s 


BIOLOGY 

LIBRARY 

6 


. 


.OUTLINES  OF 
EVOLUTIONARY  BIOLOGY 

BY 

ARTHUR    BENDY,    D.Sc.,    F.R.S., 


• 


Professor  of  Zoology  in  the   University  of  London  (King's  College)  ; 

Zoological    Secretary    of    the    Linnean    Society    of    London  ; 

Honorary  Member  of  the  New  Zealand  Institute  ;  formerly 

Professor    of    Biology    in    the    Canterbury    College 

(University  of  New  Zealand),  and  Professor  of 

Zoology  in  the  South  African  College, 

Cape  Town. 


NEW   YORK 

D.    APPLETON    AND    COMPANY 
MGMXIV 


>f 

BIOLOGY 
LIBRARY 


PEEFACE 


BIOLOGY,  the  fundamental  science  of  living  things  in  all  their 
manifold  relations,  is  a  study  which,  at  the  present  time,  is  but 
little  encouraged  by  educational  authorities  in  this  country.  It 
has  no  place  in  the  ordinary  school  curriculum  and  even  in  our 
Universities  it  has  been  thrust  into  the  background,  partly 
because  University,  authorities  devote  so  much  of  their  attention 
nowadays  to  subjects  which  are  considered  more  likely  to 
bring  in  a  direct  pecuniary  reward  to  the  student,  and  partly 
because  of  the  immense  elaboration  of  the  various  branches  of 
biological  science,  such  as  Zoology,  Botany,  Physiology,  Com- 
parative Anatomy  and  Embryology,  that  has  taken  place  in 
recent  years,  and  the  claims  of  these  to  more  or  less  separate 
recognition. 

The  student,  if  he  studies  Biology  at  all  for  its  own  sake, 
which  is  seldom  the  case,  usually  confines  himself  almost 
entirely  to  one  or  other  of  these  branches,  which  he  finds 
treated  more  or  less  as  an  independent  science,  with  an  extensive 
literature  of  its  own,  and  he  runs  a  grave  risk  of  losing  sight  of 
the  general  principles  which  underlie  all  and  from  which  all 
derive  their  chief  educational  value. 

The  medical  student,  it  is  true,  usually  takes  a  year's  course 
in  what  is  called  Biology,  but  his  curriculum  is,  perhaps 
unavoidably,  dominated  by  the  type- system  and  by  what  is 
thought  likely  to  be  of  direct  service  to  him  in  his  future 
anatomical  and  physiological  studies,  so  that  in  the  brief  time 
which  the  medical  authorities  allow  him  to  devote  to  the 
scientific  foundation  of  his  professional  work  he  has  but  little 
opportunity  for  a  philosophical  treatment  of  the  subject. 

When  we  at  length  come  to  realize  the  meaning  of  Man's 
position  as,  for  the  time  being,  the  highest  term  of  a  great 
evolutionary  series  which  stretches  far  back  into  the  dawn  of 

- 


vi  PKEFACE 

the  earth's  history,  and  to  appreciate  the  importance  of  the  fact 
that  he  derives  his  existence  from  the  same  ultimate  sources  and 
is  subject  to  the  same  natural  laws  as  all  the  other  living  things 
with  which  he  shares  the  earth,  we  shall  perhaps  see  the 
necessity  for  making  Biology,  in  the  widest  sense  of  the  term, 
one  of  the  foundation  stones  of  our  educational  system. 

In  the  meantime  those  who  wish  to  familiarize  themselves 
with  the  rapidly  accumulating  results  of  biological  investigation 
and  the  bearing  of  these  results  upon  human  problems  ought 
not  to  be  debarred  from  so  doing  by  want  of  the  necessary 
knowledge  of  fundamental  facts  and  principles,  and  it  is  largely 
with  a  view  to  the  requirements  of  such  students  that  the  present 
work  is  offered  to  the  public. 

That  even  the  elementary  study  of  biological  theory  should, 
wherever  possible,  be  preceded  by  a  systematic  course  in  Zoology 
and  Botany,  based  upon  the  type-system  and  including  laboratory 
work,  is,  no  doubt,  indisputable.  Unfortunately,  under  existing 
conditions,  regular  laboratory  work  is,  for  most  people,  im- 
possible. We  are  apt  to  forget,  however,  that  in  reality  we  all 
of  us  spend  our  lives  in  a  biological  laboratory,  where  we  are 
surrounded  by  living  organisms  which  we  can  hardly  avoid 
studying.  In  this  way  we  learn  much  of  the  nature  of  living 
things  and  are  to  some  extent  prepared  for  the  study  of  biological 
principles. 

The  problems  of  life,  however,  cannot  be  satisfactorily  solved 
if  we  confine  our  attention  to  the  higher  and  more  familiar  forms 
of  plants  and  animals.  Man,  in  particular,  is  far  too  complex  a 
type  to  begin  with  in  a  philosophical  treatment  of  the  subject. 
The  logical  method  of  study  is,  no  doubt,  to  follow  as  closely  as 
possible  the  course  which  we  believe  to  have  been  taken  in  the 
actual  evolution  of  living  things,  beginning  with  the  simple  and 
ending  with  the  complex.  This  method,  of  course,  is  attended 
with  certain  practical  difficulties,  mainly  due  to  the  microscopic 
size  of  the  more  primitive  organisms,  but  these  difficulties  are 
pot  insurmountable  and  need  not  be  considered  in  relation  to  the 
present  work. 

As  I  wish  this  book  to  be  of  use  to  those  who  have  had  no 
special  biological  training,  as  well  as  to  students  who  have  taken 
the  ordinary  first  year's  course,  I  have,  in  the  earlier  chapters, 
dealt  in  a  very  elementary  manner  with  the  structure  and 
functions  of  both  plants  and  animals.  I  have  described  Amoeba 


PREFACE  vii 


and  Hajn  ii»coccusjm  considerable  detail  and  used  these  familiat 
organisms  as  fljlson  which  to  hang  some  elementary  ideas  with 
regard  to  theJIature  of  living  things  and  the  differences  between 
animals  anjjplants.  Otherwise  I  have  as  far  as  possible  avoided 
the  type-astern  as  being  altogether  unsuitable  for  a  work  of 
this  kino;  though  of  course  I  have  been  obliged  to  refer  to 
numerous  different  organisms  in  illustration  of  special  points. 

It  was  only  by  rigidly  excluding  from  the  earlier  part  of  the 
book  everything  that  was  not  considered  essential  to  the  under- 
standing of  general  principles  that  it  has  been  possible  to  find 
space  for  even  a  brief  presentation  of  the  evidence  upon  which 
the  theory  of  organic  evolution  rests,  and  for  a  discussion  of  the 
principal  factors  which  appear  to  have  co-operated  in  determining 
the  course  of  that  evolution. 

Although  the  entire  work  is  intended  to  be  of  an  elementary 
character,  it  has  been  impossible,  in  connection  with  the  theory 
of  heredity,  to  avoid,  on  the  one  hand,  a  considerable  amount  of 
cytological  detail,  and,  on  the  other,  some  discussion  of  theoretical 
speculations  of  a  highly  controversial  nature.  In  dealing  with 
these  vexed  questions,  which  underlie  the  whole  problem  of 
organic  evolution,  I  have  endeavoured  to  present  the  views  of 
opposing  schools  of  thought  as  fairly  as  possible,  but  I  must 
confess  that  I  have  ventured  to  lay  considerable  stress  upon 
ideas  which,  though  widely  accepted  elsewhere,  have  not  as  yet 
met  with  much  appreciation  in  this  country,  though  that  they 
will  do  so  in  the  future  can  hardly  be  doubted. 

By  way  of  introduction  to  the  discussion  of  the  factors  of 
organic  evolution  a  chapter  has  been  devoted  to  the  views  of 
Buffon,  Erasmus  Darwin  and  Lamarck,  and  another  to  those  of 
Charles  Darwin,  Robert  Chambers  and  Alfred  Russel  Wallace, 
and  I  have  endeavoured  to  present  the  opinions  of  these  classical 
authors  as  far  as  possible  by  means  of  quotations  from  their 
own  writings. 

In  a  work  such  as  the  present  the  employment  of  numerous 
technical  terms  is,  of  course,  unavoidable,  but  it  is  hoped  that 
the  meaning  of  these  is  sufficiently  explained  in  the  text,  and  the 
use  of  the  index  should  obviate  any  difficulties  in  this  respect, 
especially  if  the  book  is  read  systematically  from  the  first  chapter 
911  wards. 

It  is  a  pleasure  to  record  my  thanks  to  many  colleagues  who 
have  ungrudgingly  helped  me  in  various  ways.  Amongst  these 


viii  PREFACE 

I  should  like  especially  to  mention  Professor  Poulton,  Professor 
Herbert  Jackson,  Dr.  Stapf,  Dr.  Daydon  Jackson,  Dr.  Sibly  and 
Professor  G.  E.  Nicholls  ;  while  to  Mr.  R.  W.  H.  Row,  of  the 
Zoological  Department  at  King's  College,  I  am  much  indebted 
for  the  trouble  which  he  has  taken  in  reading  through  the 
proof-sheets  and  for  many  valuable  suggestions. 

I  owe  a  sincere  acknowledgment  to  my  publishers,  Messrs. 
Constable  &  Co.,  for  their  generosity  in  the  matter  of  illustrations. 
Most  of  these  are  either  entirely  new  or,  in  some  cases,  specially 
re-drawn  from  original  memoirs.  I  have  made  extensive  use  of 
photomicrography  for  microscopic  subjects,  and  in  the  prepara- 
tion of  the  photographs  at  King's  College  have  received  much 
assistance  from  Dr.  Rosenheim,  Mr.  Row,  and  my  assistant 
Mr.  Charles  Biddolph. 

For  the  loan  of  the  blocks  from  which  the  remainder  of  the 
illustrations  have  been  printed,  or  for  permission  to  copy,  I  am 
indebted  to  the  generosity  of  the  following  : — 

Messrs.  George  Allen  and  Son, 

"  The  American  Journal  of  Science," 

Mr.  Edward  Arnold, 

Messrs.  George  Bell  and  Sons, 

Messrs.  A.  and  C.  Black, 

The  Cambridge  University  Press, 

The  Clarendon  Press, 

Messrs.  Constable  &  Co., 

Messrs.  Duckworth  &  Co., 

Herr  W.  Engelmann, 

Herr  Gustav  Fischer, 

"  The  Journal  of  Experimental  Zoology,'' 

Messrs.  Kegan  Paul,  Trench,  Triibner  &  Co., 

The  Council  of  the  Linnean  Society  of  London, 

Messrs.  Longmans,  Green  &  Co., 

Mr.  John  Murray, 

"  The  Quarterly  Journal  of  Microscopical  Science," 

The  Council  of  the  Ray  Society, 

Messrs.  Smith,  Elder  &  Co., 

The  Trustees  of  the  British  Museum, 

Messrs.  T.  Fisher  Unwin, 

Messrs.  F.  Warne  &  Co., 

Verlag  des  Bibliographischen  Instituts,  Leipzig  und  Wien. 


PREFACE  ix 

I  must  also  express  my  gratitude  to  the  numerous  authors 
whose  work  I  have  made  use  of,  and  whose  names  are  mentioned 
in  the  appropriate  places. 

I  am  indebted  to  the  Council  of  the  Royal  Society  of  Arts  for 
permission  to  make  use  of  the  Aldred  Lecture  which  I  delivered 
before  the  Society  in  1909,  and  which  is  to  a  large  extent- 
reprinted  from  their  Journal  in  Chapter  XIV. 


ARTHUR  DENDY. 


KING'S  COLLEGE,  LONDON, 
December,  1911. 


PEEFACE  TO  SECOND  EDITION. 

So  short  a  time  has  elapsed  since  the  publication  of  the  first 
edition  of  this  book  that  it  has  not  been  found  necessary  to 
make  many  alterations  in  reprinting  it.  Advantage  has  been 
taken,  however,  of  the  opportunity  afforded  to  make  a  few 
corrections  and  to  add  a  few  notes.  At  the  request  of  my 
publishers  I  have  also  added  a  Glossary  of  Technical  Terms, 
and  I  have  to  thank  Professor  Bernard  Bosanquet,  LL.D.,  for 
his  kindness  in  revising  the  proof-sheets  of  this,  with  special 
reference  to  the  Greek  and  Latin  derivations.  I  am  also  indebted 
to  the  Rev.  A.  C.  Headlam,  D.D.,  Principal  of  King's  College,  for 
valuable  advice  in  this  connection. 


ARTHUR  DENDY. 


KING'S  COLLEGE,  LONDON, 
October,  1912. 


.' 


CONTENTS 


PAET  L— THE    STRUCTURE   AND   FUNCTIONS   OP 
ORGANISMS— THE   CELL   THEORY 

CHAPTER  I 

PAGE 

Introductory:  The  nature  of  life — The  liviog  organism  viewed  as  a 
machine — The  essential  functions  of  the  living  body— The  source 
of  energy  in  living  things  .  .  .  .  .  .  .  .  1 

CHAPTER  II 
Amoeba  as  a  typical  organism— The  properties  of  protoplasm        .        .       12 

CHAPTER  III 
Haematococcus — The  differences  between  animals  and  plants        .        .      27 

CHAPTER  IV 

The  cell  theory — Unicellular  organisms — Differentiation  and  division 
of  labour — Co-operation — The  transition  from  the  unicellular  to 
the  multicellular  condition — The  early  development  of  multicellular 
animals  and  plants  ...  ...... 

CHAPTER  V 

The  cell  theory  as  illustrated  by  the  histological  structure  of  the  higher 
animals  and  plants — Limitations  of  the  cell  theory— The  cell  as 
the  physiological  unit  .  .  .  . 


CHAPTER  VI 

The  multiplication  of  cells — Mitotic  and  amitotic  nuclear  division 

PART  II.— THE   EVOLUTION  OF   SEX 

CHAPTER  VII 

Limitation  of  the  powers  of  cell-division — Rejuvenescence  by  conjuga- 
tion of  gametes — The  origin  of  sex  in  the  Protista          ,         .         .       81 


xii  CONTENTS 

CHAPTER  YIII 

PAGE 

Sexual  phenomena  in  multicellular  plants  —  The  distinction  between 
somatic  cells  and  germ  cells  —  Alternation  of  sexual  and  asexual 
generations  —  Suppression  of  the  gametophyte  in  flowering  plants  .  95 

CHAPTER  IX 

Sexual  phenomena  in  multicellular  animals  —  Structure  and  life  history 
of  Hydra  and  Obelia  —  Alternation  of  generations  —  The  ccelomate 
type  of  structure  —  Secondary  sexual  characters  —  The  evolution 
of  sex  .  ,  ......  .  .  .  .113 

CHAPTER  X 

Origin  of  the  germ  cells  in  multicellular  animals  —  Maturation  of  the 
germ  cells  —  Reduction  of  the  chromosomes  —  Sex  determination  in 
insects  —  Different  forms  of  gametes—  Mutual  attraction  of  the 
gametes  —  Fertilization  and  parthenogenesis  .  .  .  .  .129 

PAKT   III.—  VAKIATION   AND  HEREDITY 
CHAPTER  XI 

Variation  —  Meristic  and  substantive  variations  —  Fluctuations  and 
mutations—  Somatogenic  and  blastogenic  variations  —  Origin  of 
blastogenic  variations  .  .  .  .  .  .  .  .  .  148 

CHAPTER  XII 

Heredity  —  General  observations  —  Darwin's  theory  of  pangenesis  and 
Weismann's  theory  of  the  continuity  of  the  germ  plasm  —  The 
.  nucleus  as  the  bearer  of  inheritable  characters        ....     161 

CHAPTER  XIII 

The  inheritance   of  acquired  characters  and  the  mnemic  theory  of 

heredity  ............     176 

CHAPTER  XIV 

The  Mendelian  experiments  in  hybridization  —  The  doctrine  of  unit 
characters  and  the  purity  of  the  gametes  —  Galton's  law  of 
inheritance  .  .  .  .  .  .  .  .  -  .  .  194 


PART  IV.—  THE  THEORY  AND  EVIDENCE  OP 
ORGANIC  EVOLUTION:  ADAPTATION 

CHAPTER  XV  « 

Organic  evolution  versus  special  creation  —  Spontaneous  generation  and 

biogenesis  —  The  origin  of  living  things  ......     211 


CONTENTS  xiii 

CHAPTER  XVI 

PAGE 

The  continuity  of  life — The  conception  of  species — The  principles  of 

taxonomy — The  taxonomic  evidence  of  organic  evolution        .         .221 

CHAPTER  XVII  "T6" 

Connecting  links — Homology  and  analogy —Convergent  evolution — 

Change  of  function — Vestigial  structures — Reversion     .         .         .     232 

CHAPTER  XVIII  I  I 

Ontogeny — The  recapitulation  hypothesis — Interpretation  of  the  onto- 
genetic  record — Palingenetic  and  csenogenetic  characters 

CHAPTER  XIX 

The  stratified  rocks — Geological  periods — The  age  of  the  habitable  earth 
— The  geological  record — The  succession  of  the  great  vertebrate 
groups 282 

CHAPTER  XX 

Fossil  pedigrees — Ancestry  of  birds,  horses,  elephants  and  whales        .     305 

CHAPTER  XXI      ' 

Geographical  distribution — Areas  of  distribution — Barriers  to  migra- 
tion—Means of  dispersal — Changes  in  the  physical  conditions  of 
the  earth's  surface — The  evidence  afforded  by  the  study  of  geogra- 
phical distribution  with  regard  to  the  theory  of  organic  evolution  .  319 

CHAPTER  XXII 

Adaptation  to  environment  in  animals— Deep  sea  animals — The  coloura- 
tion of  animals — Protective  and  aggressive  resemblances — Warning 
colours — Mimicry — Epigamic  ornamentation  .....  334 

CHAPTER  XXIII 

Adaptation  to  the  environment  in  plants — Alpine  plants,  desert  plants 
and  lianes — The  modification  of  flowers  in  relation  to  insect- 
fertilization  ...  350 

PART  V.— FACTORS   OF  ORGANIC  EVOLUTION 

CHAPTER  XXIV 
Views  of  Buff  on,  Erasmus  Darwin  and  Lamarck 365 

CHAPTER  XXV 

Robert  Chambers  and  the  "Vestiges  of  Creation" — Natural  selection 

— The  views  of  Charles  Darwin  and  Alfred  Russel  Wallace   .        .     383 


xiv  CONTENTS 


CHAPTER  XXVI 

PAGE 

Selection  not  confined  to  the  organic  world — Illustrations  of  the  action 
of  natural  selection  in  the  struggle  for  existence — Degeneration 
— Flightless  birds — Extermination  of  the  Morioris — Sedentary 
animals — Parasites — Co-operation  of  natural  selection  and  the 
so-called  Lamarckian  factors  of  evolution  —  The  influence  of 
internal  secretions  upon  growth — Increase  in  size  beyond  the 
limits  of  utility 395 

CHAPTER  XXVII 

Artificial  selection — Continuous  and  single  selection — The  mutation 
theory  of  the  origin  of  species  —  Mutual  adaptation  —  Unit 
characters  —  Isolation  —  Physiological  selection  —  Non-adaptive 
characters — The  evolution  of  man 410 

INDEX  429 


GLOSSARY  OF  TECHNICAL  TERMS  . 


GLOSSARY  OF  TECHNICAL  TEEMS 


ABIOGENESIS    (Gr.     a,    without ;    ./3i'os,     life  ;     yeWo-u,     birth), 

spontaneous  generation,  the  origin  of  living  from  not  living 

matter. 
ACHROMATIC  FIGURE  (Gr.  a,  without;  \p^a,  colour),  that  part  of 

the  nuclear  figure  which  remains  colourless  after  treatment 

with  certain  staining  reagents. 
ALBUMEN  (Lat.  albumen,  white  of  egg),  a  proteid  substance  which 

occurs  in  the  white  of  eggs. 
ALLANTOIS  (Gr.  dAAa?,  sausage),  a  membranous  bag  which  grows 

out  from  the  alimentary  canal  in  the  embryos  of  higher 

vertebrates,  and  serves  as  an  organ  of  respiration  or  nutrition, 

or  both. 
ALLELOMORPH  (Gr.  dAA?jAa)y,  one  another ;  JUO/J^T},  kind,  sort,  form), 

one   of   a   pair    of    alternative    characters   in    Mendelian 

inheritance. 

AMBULATORY  (Lat.  ambulator,  walker),  used  in  walking. 
AMITOTIC   (Gr.  a,   without  ;    JUU'TOS,   thread),   a   term   applied  to 

nuclear  division  without  mitosis  (q.v.) . 
AMNION  (Gr.  apvlov),   a   membranous   bag   which    encloses   the 

embryo  in  higher  vertebrates. 

AMOEBOID,  a  term  applied  to  movements  like  those  of  Amoeba. 
AMPHIMIXIS   (Gr.    dju<K    on    both    sides  ;    /xtftj,    mingling),   the 

mingling  of  maternal  and  paternal  characters  in  the  fertilisa- 
tion of  the  egg. 
ANABOLISM  (Gr.  avafioXri,  throwing  up),  the  constructive  chemical 

processes  which   go  on  in  the   living  body,  whereby  food 

material  is  converted  into  protoplasm. 
ANALOGY  (Gr.  dvoAoyto),  superficial  resemblance  due  to  similarity 

of  function. 
ANATOMY  (Gr.'cba,  ro/x?y,  cutting  up),  the  study  of  the  structure 

of  organisms  (more  especially  as  revealed  by  dissection). 
ANDRCECIUM  (Gr.   avr/p,   man ;    ofaCov,   dwelling),    the   collective 

stamens  or  "  male  "  organs  of  a  flower. 


xvi  GLOSSARY  OF   TECHNICAL   TERMS 


ANEMOPHILOUS    (Gr.   ave^o?,  wind  ;    <£i'Aos,    beloved),     a    term 

applied  to  flowers  which  are  pollinated  by  wind. 
ANISOGAMY  (Gr.  awo-oj,  unequal  ;   yajuo?,   marriage),    the   union 
of   differentiated   (male    and    female)    gametes    or    sexual 
cells. 

ANTHEEIDIUM  (Gr.  avQrjpos,  pertaining  to  a  flower),  an  organ  or 
receptacle  in  which  male  sexual  cells  or  gametes  are  pro- 
duced, in  ferns  and  algae. 
ANTHEES  (Gr.  avQrjpos,  pertaining  to  a  flower),  the  receptacles 

which  contain  the  pollen  grains  in  flowering  plants. 
ANTICEYPTIC   (Gr.  dm',  opposed   to  ;    Kpvirros,   hidden),    a   term 
applied  to  colouration  adapted  for  concealment  when  used 
as  a  means  of  aggression  by  ambuscading. 
APATETIC   (Gr.  dTrarrjrtKo'j,  fallacious),     a  term  applied   to  mis- 

leading colouration,  whether  for  offence  or  defence. 
APOGAMY  (Gr.  a-no,  away  from  ;  yd/xoj,  marriage),  the  suppression 

of  the  sexual  process  in  the  reproduction  of  certain  plants. 
APOSEMATIC  (Gr.  d™,  away  from  ;  O7?jma,  signal),    a   term  applied 
to    warning    colours,     which     serve     to     frighten     away 
enemies. 

AECHEGONIUM  (Gr.  apx>],  beginning;  yoW,  fruit,  offspring),    the 

female  organ  or  receptacle  in  which  the  ovum  is  lodged  in 

certain  plants,  and   in  which  the  young  plant  begins   its 

development. 

ARCHITYPE    (Gr.    dpx*-,    primordial,    chief  ;    TVTTOS,    type),     an 

original  form  from  which  others  may  be  derived. 
AETHEOPODS  (Gr.  apBpov,  joint  ;   TTOVJ,  foot   or    limb),     animals 
with  jointed  limbs,  belonging  to  the  group  Arthropoda,  e.g., 
lobsters  and  insects. 
ARTIODACTYL   (Gr.  a/mos,  equal  ;    baKTv\os,   finger),     having    an 

even  number  of  fingers  or  toes. 

ASEXUAL  REPEODUCTION,  reproduction  without  any  sexual  process. 
ASTEE  (Gr.    dorrjp,    star),  a  star-like   figure  appearing   in  the 

cell  during  mitosis. 
ATAVISM  (Lat.  atavus,  ancestor),  a  sudden  return  on  the  part  of 

an  organism  to  an  ancestral  condition. 
AUTOMATISM  (Gr.  avTo^aros,  acting  of  one's  own  will),  spontaneous 

action,  not  in  response  to  recognisable  stimuli. 
BINOMIAL  (Lat.  binominis,  having  two  names),  a  term  applied  to 
the  Linnean  system  of  nomenclature,  in  which  each  species 
is  known  by  a  generic  and  a  specific  name, 


GLOSSARY  OF   TECHNICAL   TERMS  xvii 


BIOGENESIS  (Gr.  jftt'os,  life;  yeWo-ts,  birth),  the  origin'  of  living 

things  from  pre-existing  living  things. 
BIOGENETIC  LAW  (Gr.  (Bios,  life  ;  yeWcns,  origin),  Haeckel's  term  for 

the  recapitulation  hypothesis. 
BIOLOGY  (Gr.  /3to?,  life  ;  Ao'yoy,  discourse),  the  science  of  living 

things. 
BIOPHORS  (Gr.    £i'oj,  life;    cfyopevs,    carrier),  the  smallest  living 

units  of  which  protoplasm  is  composed  (according  to  Weis- 

mann's  theory). 
BLASTOCCEL  (Gr.  /3Aaoros,  embryo  ;  KotAoi>,  a  hollow),  the  cavity 

of  a  blastula  embryo. 
BLASTODERM  (Gr.  /3Aaoroj,  embryo  ;  ^p^a,  skin),  a  layer  of  cells 

which  in  heavily  yolked  eggs  spreads  over  the  surface  and 

gives  rise  to  the  embryo. 
BLASTOGENIC    (Gr.    /SAaoro'j,    germ  ;    rt.   yer-,  origin),    a    term 

applied  to  characters  which  originate  in  the  germ  cells. 
BLASTOMERES   (Gr.   /3Aaoro's,  embryo;    /xepos,    part),   the    undif- 

ferentiated  cells  into  which  a  developing  egg  first  divides. 
BLASTOPORE   (Gr.  /SAaoros,  embryo ;  WO/DOS,  passage),  the  mouth 

of  a  gastrula  embryo. 
BLASTOSPHERE  (Gr.  jSAaoros,  embryo  ;  o-^cupa,  sphere),  a  hollow, 

spherical  embryo  composed  of  a  single  layer  of  cells. 
BLASTOSTYLES  (Gr.   BAaoros,    embryo ;    crruAo?,  pillar),   the  rod- 
shaped  individuals  which  in  certain  hydroid  colonies  (e.g., 

Obelia)  bear  the  developing  medusae. 
BLASTULA,  another  name  for  blastosphere  (q.v.). 
BRACHIOPODS  (Gr.  8payj><*>v,  arm ;  TTOVS,  foot),  invertebrate  animals 

which  bear  a  superficial  resemblance  to  bivalve  Mollusca, 

but  belong  to  a  totally  different  group,  known  as  Brachio- 

poda ;  sometimes  called  lamp  shells. 
C^ENOGENETIC  (Gr.  Kdivos,  recent ;  yeVecris,  origin),  a  term  applied 

to  characters  of  comparatively  recent  origin. 
CAINOZOIC  (Gr.  KCLIVOS,  recent ;  £0077,  life),  a  term  applied  to  the 

most  recent   of  the  main  divisions  of  the  earth's  history, 

during  which   the   more  modern   forms   of    living   things 

appeared. 
CALYX  (Gr.  KaAuf,  calyx),  the  outermost  of  the  whorls  of  modified 

leaves  of  which  a  typical  flower  is  composed. 
CAMBIUM  (late  Lat.,  exchange),  a  layer  of  cells  which  lies  between 

the  wood  and  the  bast  and  which,  by  cell  division,  causes 

increase  in  the  thickness  of  both. 

B.  b 


xviii  GLOBSABY  OF   TECHNICAL  TERMS 

CAMBRIAN,  the  first  epoch  of  the  palaeozoic  era,  the  characteristic 
rocks  of  which  are  largely  developed  in  North  Wales 
(Cambria). 

CARBOHYDRATES,  a  group  of  chemical  compounds  which  contain 
the  elements  carbon,  hydrogen  and  oxygen,  the  two  latter  in 
the  proportion  H20 ;  e.g.,  starch,  sugar. 

CARBON  DIOXIDE  (  =  carbonic  acid  gas),  a  gaseous  compound  of 
carbon  and  oxygen,  having  the  formula  C02. 

CARBONIFEROUS  (Lat.  carlo,  coal ;  ferre,  to  bear),  one  of  the 
palaeozoic  epochs,  during  which  coal  was  deposited. 

CARPALS  (Gr.  nap-nds,  wrist),  the  bones  of  the  wrist. 

CARPELS  (Gr.  Kapiros,  fruit),  the  modified  leaves  which  form  the 
so-called  ovary  of  a  flower,  from  which  the  fruit  arises. 

CASEIN  (Lat.  caseus,  cheese),  a  proteid  substance  found  in 
cheese. 

CASTRATION  (Lat.  castrare,  to  castrate),  the  removal  of  the  male 
gonads  or  testes. 

CELL  (Lat.  cella,  apartment,  cell),  a  nucleated  protoplasmic  unit 
(see  p.  86  et  seq.  for  origin  of  the  term). 

CELLULOSE,  the  substance  of  which  vegetable  cell  walls  are  usually 
composed ;  a  carbohydrate. 

CENTROSOME  (Gr.  KeVr/aoz/,  centre ;  <ro>/*a,  body),  a  minute  proto- 
plasmic body  concerned  in  the  mechanism  of  nuclear  division. 

CENTROSPHERE  (Gr.  Ktvrpov,  centre ;  a-tyaipa,  sphere),  a  minute 
differentiated  mass  of  protoplasm  immediately  surrounding 
the  centrosome  (q.v.). 

CERCARIA  (Gr.  K€>KOS,  tail),  the  tailed  larva  of  a  parasitic 
fluke. 

CHELA  (Gr.  X'jMj  claw),  a  claw-shaped  siliceous  spicule  found  in 
certain  sponges. 

CHEMOTAXIS  (Gr.  raft?,  arrangement),  a  movement  or  arrange- 
ment in  response  to  a  chemical  stimulus. 

CHLOROPHYLL  (Gr.  x\a>p6s,  green ;  <f>v\\ov,  leaf),  the  characteristic 
green  colouring  matter  of  plants. 

CHLOROPLASTIDS  (Gr.  x^P'^,  green  ;  TrAaorr??,  modeller),  proteid 
bodies  (contained  within  cells)  in  which  chlorophyll  is 
deposited. 

CHORDATE  (Gr.  x°P8^  cord),  provided  with  a  notochord  (q.r.). 

CHROMATIN  (Gr.  xP^a>  colour),  a  constituent  of  the  nucleus 
which  becomes  stained  by  certain  dyes  and  which  is  sup- 
posed by  many  to  be  tho  bearer  of  inheritable  tendencies. 


GLOSSARY  OF   TECHNICAL   TEEMS  xix 

CHROMATOPHORES  (Gr.  XP^M**,  colour;  <f>opevs,  carrier)?  proteid 
bodies  (contained  within  cells)  in  which  colouring  matter  is 
deposited. 

CHROMOMERES  (Gr.  XP^Ma>  colour ;  pepos,  part),  the  constituent 
parts  of  chromosomes  (#.r.);  termed  by  Weismann  "ids." 

CHROMOSOMES  (Gr.  XP^M«,  colour ;  o-co/txa,  body),  definite  aggrega- 
tions of  chrornatin  which  appear  during  mitotic  nuclear 
division  ;  termed  by  Weismann  "  idants." 

CILIA  (Lat.  cilium,  eyelash),  hair-like  protoplasmic  projections 
from  the  surface  of  a  cell,  which  have  the  power  of  rapid, 
co-ordinated  vibration. 

CLAVICLE  (Lat.  claricula,  small  key),  one  of  the  bones  of  the 
shoulder  girdle,  the  collar-bone  in  man. 

CLEAVAGE,  the  division  of  the  developing  egg  into  undifferentiated 
cells  or  blastomeres. 

COCCYX  (Gr.  KOKKV£,  cuckoo),  the  lower  end  of  the  vertebral 
column  in  man. 

CCELENTERATE  (Gr.  KotAov,  cavity ;  tvrcpov,  gut),  the  term  applied 
to  those  animals  (Coelenterata)  in  which  there  is  only  one 
cavity  in  the  body,  the  enteron  or  digestive  cavity. 

CCELOM  (Gr.  KOI\OV,  cavity),  the  body  cavity,  which  lies  between 
the  gut  and  the  body  wall  in  ccelomate  animals. 

CQSLOMATE,  possessing  a  body  cavity  or  coBlom  (y.?.-A 

COLLOIDS  (Gr.  /co'AAa,  glue),  gummy~substances  which  will  not 
diffuse  through  organic  membranes. 

CONGENITAL  (Lat.  congenitatis,  present  at  birth),  a  term  applied 
to  characters  present  at  birth  (but  not  necessarily 
blastogenic). 

CONJUGANT  (Lat.  conjugare,  to  join  together),  one  of  two  "con- 
jugating" individuals  (in  Paramoecium,  &c.). 

CONJUGATION  (Lat.  conjugare,  to  join  together),  the  union  of  two 
sexual  cells  or  gametes  to  form  a  zygote,  which  is  capable  of 
developing  into  a  new  individual  (the  term  is  sometimes 
limited  to  cases  of  isogamy,  but  is  used  in  this  work  in  a  more 
general  sense). 

CONVERGENCE,  the  acquisition,  by  different  organisms,  of  similar 
form  or  structure  as  the  result  of  a  similar  mode  of  life, 
and  not  as  the  result  of  inheritance  from  common 
ancestors. 

CORACOID  (Gr.  Kopa£,  raven  ;  etSoj,  form ;  anything  hooked  like  a 
raven's  beak),  one  of  the  bones  of  the  typical  shoulder  girdle 

62 


xx  GLOSSARY  OF   TECHNICAL   TERMS 

COROLLA  (Lat.,  little  wreath),  one  of  the  whorls  of  modified 
leaves  of  which  a  flower  is  composed,  usually  formed  of 
gaily  coloured  petals. 

CORPORA  LUTE  A  (Lat.,  yellow  bodies),  glandular  structures  which 
make  their  appearance  on  the  surface  of  the  ovary  (in 
mammals)  at  the  spots  whence  ova  have  been  discharged. 

CORPUSCLE  (Lat.  corpusculum,  little  body),  a  term  applied  to 
certain  cells,  such  as  blood  corpuscles. 

CORRELATION,  the  dependence  of  one  character  upon  another. 

CORTICAL  (Lat.  cortex,  bark,  rind),  belonging  to  a  protective  outer 
layer. 

COSMOPOLITAN  (Gr.  KO'O-JUIO?,  world ;  TroAiTrj?,  citizen),  of  world- 
wide distribution. 

COSMOZOA  (Gr.  Mo-pas,  universe  ;  C<j>oy,  living  thing),  minute 
germs  which  are  supposed  to  float  about  in  space  between 
the  planets  (purely  imaginary). 

COTYLEDONS  (Gr.  KoruArjSoVes,  suckers),  the  seed-leaves  of  flowering 
plants. 

CRETACEOUS  (Lat.  creta,  chalk),  a  term  applied  to  the  geological 
period  in  which  the  chalk  rocks  were  deposited. 

CRYPTIC  (Gr.  KpuTiroy,  hidden),  a  term  applied  to  colouration  when 
adapted  for  concealment. 

CRYSTALLOIDS  (Gr.  KpvoraAAo?,  crystal ;  eiSoj,  form),  a  group  of 
chemical  substances  which  usually  form  crystals  and  which 
are  capable  of  diffusing  through  organic  membranes. 

CYCLOPEAN  (Gr.  KVKX^,  cyclops),  having  a  single  eye  in  the 
middle  of  the  head. 

CYCLOSTOME  (Gr.  Kv/cAos,  circle;  oro/mo,  mouth),  a  term  applied 
to  the  lampreys  on  account  of  their  suctorial  circular 
mouths. 

CYST  (Gr.  KVO-TTI,  bladder,  bag),  a  protective  envelope  formed 
around  an  organism  during  a  period  of  rest. 

CYTOLOGY  (Gr.  /euros,  a  hollow  vessel  [cell]  ;  Ao'yos,  discourse), 
the  special  study  of  cells. 

CYTOLYSIS  (Gr.  KUTOS,  cell ;  Aim?,  breaking  up),  the  disintegra- 
tion of  cells. 

CYTOPLASM  (Gr.  KVT-OJ,  cell ;  TrAa^a,  lit.  that  which  is  moulded, 
used  in  the  sense  of  the  formative  material),  the  proto- 
plasm of  the  cell  body,  outside  the  nucleus. 

CYTOSTOME  (Gr.  KVTOS,  cell  ;  oro'/xa,  mouth),  the  mouth  of  a 
unicellular  organism. 


GLOSSARY   OF   TECHNICAL   TERMS  xxi 

CYTOTAXIS  (Gr.  KVTOS,  cell ;  raft?,  arrangement),  a  movement  or 

arrangement  of  cells  in  response  to  stimulation  by  other 

cells. 

CYTOTROPISM  (Gr.  KVTOS,  cell ;  17)07717,  turning),  see  cytotaxis. 
DEGENERATION,  simplification  of  structure  by  reduction  of  parts. 
DENDRON  (Gr.  btvbpov,  tree),  a  branching  outgrowth  of  a  nerve 

cell. 

DENTITION  (Lat.  dens,  tooth),  tooth  arrangement. 
DENUDATION  (Lat.  denudare,  to  lay  bare),  the  wearing  away  of 

rocks  by  atmospheric  and  other  agencies. 
DERMATOGEN   (Gr.  Stpua,  skin ;  rt.  ytv-,  origin),  the  embryonic 

layer  which,  in  higher  plants,  gives  rise  to  the  epidermis. 
DETERMINANTS,  hypothetical   bodies   of   ultramicroscopical   size, 

which  are  supposed  to  exist  in  the  chromatin  substance  of 

the  nucleus  and  to  be  responsible  for  the  transmission  of 

inheritable  characters. 
DEUTOPLASM  (Gr.  Sevrepos,  secondary ;  ?rAao>ia,  formative  material), 

the  food  material  or  yolk  stored  in  an  egg. 
DEVONIAN,  the  name  given  to  the  geological  period  in  which  the 

old  red  sandstone  was  deposited. 
DIATOMS   (Gr.  819,  double;  aro/xos,    lit.    indivisible,  used   in    the 

sense  of  "  atom  "),  a  group  of  unicellular  plants  provided 

with  double  siliceous  envelopes. 
DICHOGAMY  (Gr.  St'xa,  in  two;    ya/xos,  marriage),  maturation  of 

the  stamens  and  pistil  of  a  flower  at  different  times,  prevent- 
ing self-fertilisation. 
DIHYBRIDISM  (Lat.  duo,  two ;  hybrida,  hybrid),  a  Mendelian  term 

for  a  cross  between  organisms  which  differ  as  regards  two 

pairs  of  contrasted  characters. 

DIMORPHIC  (Gr.  6t'y,  double ;  popfai,  form),  having  two  forms. 
DIOZCIOUS  (Gr.  8ty,  double ;    oUiov,  dwelling   place),  having  the 

male  and  female  organs  in  separate  individuals. 
ECHINOIDS  (Gr.  extuo?,  hedgehog ;  etSo?,  form),  sea-urchins. 
ECTODERM  (Gr.  CKI-OS,  outside ;  5ep/ua,  skin),  the  outer   layer  of 

cells  in  a  multicellular  animal. 
ECTOPLASM  (Gr.  e/cros,  outside  ;  -nXa-v^a,  formative  material),  the 

outer  part  of  the  protoplasm  in  cells. 
EDENTATES  (Lat.  edentare,  to  knock  out  the  teeth),  a  group  of 

mammals  without  teeth  in  the  front  part  of  the  jaw. 
EMBRYO  (Gr.  eju/3puoi/),  a  young  organism  in  an  early  stage  of 

its  development. 


xxii  GLOSSARY  OF   TECHNICAL   TERMS 


EMBRYOLOGY  (Gr.  fyfipvov,  embryo  ;  Ao'yoj,  discourse),  the  study 

of  the  development  of  organisms. 
EMBRYO-SAC,  a  chamber  in  the  ovule  or  immature  seed,  in  which 

the  embryo  plant  begins  its  development. 
ENDODERM   (Gr.   Zvbov,  within  ;  6e'p/*a,  skin),  the  inner  layer  of 

cells  in  a  multicellular  animal. 
ENDOPLASM  (Gr.  tvbov,  within  ;  7rAao-^ar4oi'iiLtCtrve  material),  the 

inner  part  of  the  protoplasm  in  cells. 
ENDOSARC  (Gr.  ej;<W,  within  ;  o-ap£,  flesh),  the  inner  part  of  the 

protoplasm  in  Amoeba,  &c. 
ENDOSPERM  (Gr.  tvbov,  within  ;  o-Trepjua,  seed),  nutrient  substance 

stored  within  a  seed,  around  the  embryo. 
ENGRAM  (Gr.  cv,  in,  upon  ;  ypajujua,  something  written  or  drawn), 

a   term  used  in   the   mnemic  theory   of   heredity   for   an 

impression    made    upon     a     germ    cell     and     capable    of 

giving  rise   to  a  corresponding   character   at  some  future 

time. 
ENTELECHY  (Gr.  eiaeAe'xeia,  perfection  or  total  reaction),  a  kind  of 

guiding  principle,  by  which  it  is  attempted  to  explain  life. 
ENTERON    (Gr.    Ivrepor,     intestines),     the    digestive    cavity    in 

embryos  and  simple  animals. 
ENTOMOPHILOUS  (Gr.  oro/ios,  cut  up,  =  insect  ;  c/uAos,    beloved), 

^tp.rm  applied  to  flowers  which  are  pollinated  by  insects. 
ENUCLEATE,  deprived  of  nucleus. 
EOCENE  (Gr.  ??w?,  dawn;  KCU.VOS,  recent),  the  first  subdivision  of 

the  cainozoic  period  in  the  earth's  history. 
EPIBLAST   (Gr.   fat,  upon  ;    /SAaoros,   germ),   the  outer   layer  of 

cells  in  the  animal  embryo. 
EPICORACOID  (Gr.  CTTI,  upon  ;    Ko'paf   [coracoid]  ),  one  of  the  bones 

of  the  shoulder  girdle  in  reptiles,  &c. 
EPIDERMIS  (Gr.  eTuSe/apu's  :  fat,  upon  ;  6ep/xa,  skin),  the  outer  layer 

of  the  skin. 
EPIGAMIC   (Gr.  fat,  upon;    yafxos,  marriage),  a  term  applied  or 

colouration  which  serves  as  a  sexual  attraction. 
EPIGENESIS   (Gr.    fat,    upon  ;    yeVetn?,   origin),    the   theory    of 

development  by  the  differentiation  of  previously  undifferen- 

tiated  protoplasm  ;  opposed  to  "  p  reformation." 
EPISEMATIC   (Gr.   fat,  upon;    (rij/ua,  signal),    a   term  applied   to 

colouration  which  aids  in  recognition. 
EPITHELIUM  (Gr.  fat,  upon  ;  0r/A??,  teat),  a  layer  of  cells  covering 

a  surface. 


GLOSSAEY  OF   TECHNICAL   TERMS  xxiii 


EUCALYPTS  (Gr.  eS,  well  ;    /caXvTrro'?,  covered  ;  so-called  from  the 

flower  bud,  covered  by  the  calyx),  trees  of  the  genus  Euca- 

lyptus, found  in  Australia. 
EXCRETION  (Lat.  excretus,  separated  out),  the  function  by  which 

waste  products  are  got  rid  of. 
FEMUR  (Lat.,  thigh),  the  thigh  bone. 
FIBULA  (Lat.,  pin),  one  of  the  bones  of  the  leg. 
FISSION   (Lat.  fissio,   a   cleaving),    the  division   of    a   cell   into 

(usually)  two  daughter  cells. 
FLAGELLUM  (Lat.,  whip),  a  whip-like  protoplasmic  projection  from 

the  body  of  a  cell,  capable  of  active  movement. 
FLUKES,  a  group  of  parasitic  worms  (Trematoda). 
FCETAL  MEMBRANES,   sac-like  membranes  which   serve    for    the 

protection,  respiration  and  nutrition  of  the  embryo  in  reptiles, 

birds  and  mammals. 
FCETUS    (Lat.,  young  animal),  a  term   usually   applied    to   the 

partially  developed  young  of  mammals,  while  still  within 

the  womb. 
FUNICULUS  (Lat.,  little  cord),  the  stalk  by  which  the  ovule  or 

seed  is  attached  to  the  wall  of  the  so-called  ovary  in  flowering 

plants. 
GAMETES  (Gr.  ya/Lie'njs,  spouse),  sexual  cells  (g^rm^cgjls)  which 

unite  in  pairs   to  form  zygotes,  which  develop   iflfo  new 

organisms. 
GAMETOGENESIS  (Gr.  ya^eVr??,  spouse  ;  ye'reo-is,  origin),  the  process 

by  which  gametes  originate  in  the  parent  organism. 
GAMETOPHYTE  (Gr.   ya/xerr/?,  spouse  ;    QVTOV,  plant),  the   sexual 

or  gamete  -producing  generation,  which  alternates  with  the 

asexual  sporophyte  in  many  plants. 
GAMOBIUM  (Gr.  ya//os,  marriage  ;  /3tos,  life),  the  sexual  generation, 

which  alternates  with  an  asexual  generation  (agamobium) 

in  many  organisms. 

GANGLION  (Gr.  ydyyXiov,  swelling),  an  aggregation  of  nerve  cells. 
GANOIDS  (Gr.  ydvos,  sheen  ;   etSos,  form),  an   ancient   group   of 

fishes,  usually  with  polished  scales. 
GASTR^A  (Gr.  yaorrjp,  paunch,   belly),  a  hypothetical   ancestral 

animal  resembling  the  gastrula  embryo. 
GASTRAL  CAVITY  (Gr.  yao-n/p,  belly),  digestive  cavity. 
GASTRULA  (Gr.    yaor?/p,  belly),  a  stage  in    the  development  of 

multicellular   animals  in  which  the  embryo  consists   of  a 

two-layered  sac,  with  a  digestive  cavity  and  a  single  opening. 


xxiv  GLOSSAEY  OF   TECHNICAL   TERMS 

GASTRULATION  (Gr.  yao-nj/o,  belly),  the  process  by  which  the  gastrula 

is  formed. 

GENUS  (Gr.  yeVo?,  race),  a  group  of  closely  related  species. 
GERM  CELLS,  single  cells  which  are  capable  of  developing  into  new 

individuals  (usually  after  conjugating  in  pairs). 
GERMINAL  DISK,  a  speck  of  active  protoplasm  in  a  heavily-yolked 

egg,  from  which  the  embryo  develops. 
GERM  LAYER,  one  of  the  fundamental  layers  of  cells  which  appear 

at  an  early  stage  of  development  in  multicellular  animals. 
(JERM   PLASM,  that   part   of   the    protoplasm   of  the  germ   cell 

(probably  the  chromatin  substance  of  the  nucleus),  which 

is  supposed  to  be  the  bearer  of  inheritable  tendencies. 
GLAND  (Lat.  glans,  nut),  an  organ  which  secretes  or  excretes  some 

special  substance. 

GLIADIN  (Gr.  yXota,  glue),  a  proteid  substance  occurring  in  flour. 
GLUCOSE  (Gr.  yAvKvs,  sweet),  a  carbohydrate,  also  known  as  grape 

sugar. 
GLUTININ  (Lat.  gluten,  glue),  a  proteid  substance  occurring  in 

flour. 
GONAD  (Gr.  yoVos,  reproduction),  the  organ  (ovary  or  testis)  in 

which  the  germ  cells  appear. 
GONODUCT  (Gr.  yoVoj,  reproduction ;  Lat.  ductus,  conduit),  a  duct 

or  tube  through  which  germ  cells  pass  to  the  exterior  of  the 

body. 

GONOPHORE  (Gr.  yoVos,  reproduction  ;  <f>op€vs,  bearer),  an  indivi- 
dual which  bears  gonads  or  reproductive  organs  (in  Hydroids). 
GONOTHECA  (Gr.  yoyos,  reproduction ;  #77*77,  box),  a  hollow  skeletal 

structure  containing  gonophores  (in  Hydroids). 
GYNCECIUM  (Gr.  <yu^,  woman;  cuaW,  dwelling  place),  the  "female  " 

organs  of  a  flower,  consisting  of  ovary,  style  and  stigma, 

formed  from  modified  leaves  known  as  carpels. 
H^MATID  (Gr.  at/xa,  blood),  a  red  blood  corpuscle. 
H^MATOCHROME  (Gr.  atjua,  blood ;  xP<»Ma>  colour),  a  red  modifi- 
cation of  chlorophyll,  found  in  Haematococcus. 
HEMOGLOBIN   (Gr.    atjua,    blood  ;    globulin,   a  proteid),    the   red 

colouring  matter  of  blood. 
HERMAPHRODITE    (Gr.   'Ep/xr??,    'A(/>podir?7,   a   god   and    goddess), 

bearing  both  male  and  female  reproductive  organs. 
HETEROGENY  (Gr.  erepoy,  different ;  rt.  ytv-,  birth),  an  alternation 

of  normal  sexual  with  parthenogenetic  individuals  in  the  life 

cycle. 


GLOSSAKY  OF   TECHNICAL   TEEMS  xxv 

HETEROSPOROUS   (Gr.  ere/oo?,  different;  airopos,   seed),   producing 

two  kinds  of  spores. 
HETEROSTYLED  (Gr.  erepos,  different ;  crruAoj,  pillar),  a  term  applied 

to  certain  plants  (Primula)   which  produce  two    kinds  of 

flowers,  differing  in  the  length  of  the  style. 
HETEROZYGOTE  (Gr.  erepo?,  different ;   fuycoro's,  joined  together),  a 

Mendelian  term  applied  to  an  organism  produced  by  the 

union  of  two  gametes  which  differ  as  regards  some  particular 

alternative  character  which  they  bear. 
HEXADACTYLISM   (Gr.   ef,    six;    baKTvXos,   finger),  a  condition  in 

which  six  fingers  or  toes  are  present  instead  of  the  normal 

five. 
HISTOLOGY  (Gr.  laros,  web ;  Ao'yos,  discourse),  the  science  which 

deals  with  animal  and  vegetable  tissues  or  cell-aggregates. 
HOLOPHYTIC  (Gr.  oAor,  altogether ;  </>uroV,  plant),  a  term  applied 

to   the   typical  vegetable  method   of  nutrition,   by   aid  of 

chlorophyll. 
HOLOZOIC  (Gr.  6'Aoz/,  altogether;  fwor,  animal),  a  term  applied  to 

the  typical  animal  method  of  nutrition,  by  means  of  already 

elaborated  food  material. 
HOMOLOGY  (Gr.  ojuoAoyia,  agreement),  essential  structural  identity 

due  to  community  of  descent. 
HOMOPLASY  (Gr.  epos,  the  same  ;   TrAeWo),  I  mould),  secondary 

resemblance  due  to  similarity  of  habit. 
HOMOSPOROUS  (Gr.  6>oy,  the  same  ;  o--o'/>cs,  seed),  producing  spores 

of  one  kind  only. 
HOMOZYGOTE  (Gr.  ojuoj,  the  same ;  (yyuros,  yoked),  a  Mendelian 

term  applied  to  an  organism  produced  by  the  union  of  two 

gametes  which  resemble  one  another  as  regards  some  par- 
ticular alternative  character  which  they  bear. 
HORMONE  (Gr.   op^v,  that  which  sets  in  motion),  an  internal 

secretion  which  circulates  in  the  body  and  acts  as  a  stimulus. 
HUMERUS  (Lat.  humerus),  the  bone  of  the  upper  arm. 
HYBRIDISATION  (Lat.  hybrida,  hybrid),  the  crossing  of  two  distinct 

varieties  or  species  by  the  union  of  the  male  gametes  of  the 

one  with  the  female  gametes  of  the  other. 

HYDRA  (Gr.  vbpa,  a  water  serpent),  the  common  fresh-water  polype. 
HYDRANTH  (Gr.  vbpa,  water  serpent ;  av6os,  flower),  a  flower-like 

nutritive  individual  of  a  hydroid  colony. 
HYDROCAULUS  (Gr.  vbpa,  water  serpent ;  KauAo's,  stalk),  the  stem 

of  a  hydroid  colony. 


xxvi  GLOSSARY  OF   TECHNICAL   TERMS 

HYDROID  (Gr.  vbpa,  water  serpent ;  etSoj,  form),  Hydra-like. 
HYDROTHECA   (Gr.    vbpa,   water   serpent ;    #77/07,   box),  a   cup-like 

skeletal  structure  containing  a  hydranth. 
HYDROZOON  (Gr.  vbpa,  water  serpent ;   faJov,  animal),  an  aquatic 

animal  of  the  group  to  which  Hydra  belongs. 
HYPERPHALANGY  (Gr.  vircp,  above ;  c/mAayf ,  phalanx),  a  condition 

in  which  the  number  of  phalanges  in  the  digits  is  increased, 

as  in  some  whales. 
HYPERTONIC  (Gr.  virep,  above ;   roWs,  tone),  a   term   applied   to 

solutions  in  which  the  osmotic  pressure  has  been  raised  by 

addition  of  salts,  &c. 
HYPOBLAST   (Gr.    VTTO,    below ;    jQAaorJs,    germ),   the   innermost 

layer  of  cells  in  the  embryo  of  a  multicellular  animal. 
HYPOSTOME  (Gr.  VTTO,  below ;  oro'/xa,  mouth),  the  part  below  the 

mouth  in  a'hydroid  animal. 
ID  (Gr.  iSioj,  personal,  peculiar),  a  complete  ancestral  germ-plasm, 

.    a  chromomere  (Weismann). 
IDANT   (Gr.    iSto?,   personal,  peculiar),  an  aggregation  of  ids,  a 

chromosome  (Weismann). 
IDIOPLASM    (Gr.    Ibios,    personal,    peculiar ;    TrXdofjia,    formative 

material),    the    hereditary    substance    of     any     particular 

cell. 
INFLORESCENCE  (L&i.florescere,io  begin  to  flower),  the  arrangement 

of  flowers  on  the  parent  plant.     •> 
INTEGRATION  (Lat.  integer,  entire),  the  process  by  which  individuals 

of  a  lower  order  become  united  to  form  an  individual  of  a 

higher  order. 
INTERCELLULAR  (Lat.  inter,  between ;    cellula,  a   small   chamber 

[cell] ),  lying  between  cells. 
INTERCLAVICLE,  one  of  the  bones  of  the  shoulder  girdle  in  reptiles 

and  monotremes. 

INTERSTITIAL,  occupying  interstices  or  gaps. 
INTUSSUSCEPTION  (Lat.  intus,  within ;  suscipere,  to  take  up),  the 

addition  of  new  particles  throughout  the  entire   mass   (of 

protoplasm),  resulting  in  growth. 
INVAGINATION  (Lat.  in,  in;  vagina,  sheath),  the  pushing  of  one 

part  of  a  hollow  body  into  the  other,  so  that  part  of  the 

original  outer  surface  becomes  internal. 
INVERTEBRATE  (Lat.  in-,  not ;  vertebra),  without  a  backbone. 
ISOGAMY  (Gr.  60-os,  equal ;    yajuo?,  marriage),  the  conjugation  of 

similar,  mutually  undifferentiated  gametes. 


GLOSSARY  OF   TECHNICAL   TERMS  xxvii 

JURASSIC,   one   of    the   subdivisions   of    the   mesozoic   era,    the 

characteristic  rocks  of  which  are  greatly  developed  in   the 

Jura  mountains. 
KABYOGAMY  (Gr.  Kapvov,  nut,  used  for  nucleus ;  ydjuos,  marriage), 

the  fusion  of  nuclei  in  conjugation. 
KARYOKINESIS   (Gr.    Kapvov,    nut,    nucleus ;    KU^CTIJ,   movement), 

another  term  for  mitosis  (q.v.). 
KARYOPLASM  (Gr.  Kapvov,  nut,  nucleus ;  TrAdcr/xa,  formative  material), 

the  protoplasm  of  the  nucleus. 
KARYOSOME   (Gr.  Kapvov ,  nut,   nucleus ;    o-oijita,   body),   a   special 

aggregation  of  chromatin  in  the  resting  nucleus. 
KATABOLISM  (Gr.  Kara^oArj,  a  throwing  down),  a  term  applied  to 

all  the  destructive  chemical  processes  which  go  on  in  the 

living  body,  resulting  in  the  formation  of  waste  products. 
LABYRINTHODONTS  (Gr.    \afivpivdos,    labyrinth  ;   obovs,    tooth),   a 

group  of  extinct  amphibians,  so-called  from  the  pattern  of 

the  teeth. 
LARVA  (Lat.  larva,  ghost,  mask),  an  immature  but  more  or  less 

active  stage  in  the  development  of  certain  animals,  differing 

widely  in  appearance  from  the  adult. 
LEGUMIN  (Lat.  leyumen,  pulse),  a  proteid  substance  found  in  peas 

and  beans. 

LEUCOCYTE  (Gr.  \CVKOS,  white ;  KVTOS-,  cell),  a  white  blood  corpuscle. 
LININ  (Gr.  \LVOV,  thread),  an  achromatic  substance  which  forms 

a  network  of  threads  in  the  nucleus. 
LYSIN    (Gr.   ATATI?,   breaking   up),  a   chemical   substance   which 

brings  about  the  disintegration  of  cells. 
MALIC  ACID  (Lat.  malum,  apple),  an  organic  acid  found  in  sour 

apples  and  other  fruit. 
MAMMAL  (Lat.  mamma,  breast),  a  vertebrate  animal  which  suckles 

its  young. 
MANUBRIUM  (Lat.,  handle),  the  projection   on  which  the  mouth 

is  placed  in  a  jelly  fish. 
MEDUS/E    (Lat.    Medusa,    the   Gorgon),    jelly   fish,    the    sexual 

individuals  of  certain  hydroids. 
MEGAGAMETES  (Gr.  /xe'yas,  large ;  ya/xeY???,  spouse),  the  large  (female) 

gametes,  where  the  latter  are  differentiated  into  two  sizes. 
MEGALECITHAL  (Gr.  jue'ya?,  large ;   \c*i0os,  yolk,  lit.  porridge),  a 

term  applied  to  eggs  which  contain  much  yolk. 
MEGANUCLEUS  (Gr.  ^yas,  large  ;  Lat.  nucleus,  kernel),  the  larger 

(somatic)  nucleus  in  Paramcecium,  &c. 


xxviii  GLOSSARY  OF   TECHNICAL   TERMS 

MEGASPORANGIUM  (Gr.  jue'ya?,  large  ;  o-no^os,  seed ;  ayyt'iov,  vessel), 
a  large  sporangium,  in  cases  where  the  latter  are  differentiated 
into  two  sizes. 

MEGASPORE  (Gr.  /xeyas,  large  ;  O-TTO/JOS,  seed),  a  large  spore,  in  cases 
where  large  and  small  are  produced. 

MEIOSIS  (Gr.  /^etWis,  reduction),  the  reduction  of  the  chromo- 
somes which  takes  place  in  the  maturation  of  germ  cells. 

MERISTEM  (Gr.  /ute/no-rrjj,  a  divider),  an  un differentiated  tissue 
in  which  cell-division  is  going  on  actively  (in  plants). 

MERISTIC  (Gr.  jue/Horrj?,  a  divider),  numerical  (variation). 

MEROGONY  (Gr.  pepos,  part ;  yoVos,  reproduction),  the  develop- 
ment of  enucleated  eggs,  after  fertilization. 

MESENTERY  (Gr.  //eVoj,  middle  ;  tvrcpov,  intestines),  the  mem- 
brane which  supports  the  intestines  in  the  body  cavity. 

MESOBLAST  (Gr.  pea-os,  middle ;  /3Aaoro's,  germ),  the  middle  one  of 
the  three  layers  of  cells  of  which  the  body  of  a  ccelomate 
embryo  is  made  up. 

MESODERM  (Gr.  /ueW,  middle;  8e/>jma,  skin),  that  part  of  the 
body  which  is  derived  from  the  embryonic  mesoblast. 

MESOGLCEA  (Gr.  /ueVo?,  middle ;  yAoia,  glue),  the  gelatinous 
supporting  layer  between  ectoderm  and  endoderm  in 
coalenterates. 

MESOPHYLL  (Gr.  /ueo-oy,  middle ;  tyvXkov,  leaf),  the  middle  layer  of 
a  leaf. 

MESOZOIC  (Gr.  /xeVo?,  middle ;  C<ov,  life),  one  of  the  primary  divi- 
sions of  the  earth's  history. 

METABOLISM  (Gr.  juera/3oA?i,  change),  the  sum  total  of  the  chemical 
processes  which  go  on  in  the  living  body. 

METACARPALS  (Gr.  juera,  next  after ;  Kopiros,  wrist),  the  bones  in 
the  palm  of  the  hand. 

METAGENESIS  (Gr.  //era-,  expressing  change ;  yeVe<m,  birth),  the 
alternation  of  sexual  and  asexual  generations. 

METAMERIC  (Gr.  /uera,  next  after ;  |ue>os,  portion),  a  term  applied  to 
a  type  of  segmentation  in  which  the  similar  parts  (metameres) 
of  which  the  body  is  composed  follow  after  one  another  in 
longitudinal  series. 

METAMORPHIC  (Gr.  /xcra-,  expressing  change ;  M°P<£7?,  form),  a 
term  applied  to  rocks  which  have  undergone  extensive 
changes  since  they  were  laid  down. 

METAMORPHOSIS  (Gr.  /mera/xop^axrij,  transformation),  a  more  or 
less  abrupt  change  from  one  stage  of  development  to  another 


GLOSSAKY  OF   TECHNICAL  TERMS  xxix 

METAPODIALS  (Gr.  /uera,  next  after ;  TTOVS,  foot),  a  collective  term 

for  metacarpals  and  metatarsals. 
METATAKSALS  (Gr.  juera,  next  after ;    Tapaos,  foot    [ankle] ),   the 

bones  of  the  instep. 

MICROGAMETES  (Gr.  fjiLKpos,  small ;  ya/xerrys,  spouse),  the  small 
(male)  gametes,  where  the  latter  are  differentiated  into  two 
sizes. 

MICROLECITHAL  (Gr.  fjuKpos,  small ;  \tKiOos,  yolk),  a  term  applied 
to  eggs  with  little  yolk. 

MICRONUCLEUS  (Gr.  fJUKpos,  small ;  Lat.  nucleus,  kernel),  the  smaller 
(germ)  nucleus  in  Paramoecium,  &c. 

MICROPYLE  (Gr.  piKpos,  small ;  uvAr;,  gate),  a  small  opening  in 
the  integuments  of  an  ovule,  through  which  the  pollen  tube 
usually  enters. 

MICROSPORANGIUM  (Gr.  /UK/JO'S,  small ;  sporangium),  a  small  sporan- 
gium, in  cases  where  the  latter  are  differentiated  into  two  sizes. 

MICROSPORE  (Gr.  nLKpos,  small ;  o-Tropo?,  seed),  a  small  spore,  in 
cases  where  large  and  small  are  produced. 

MICROZOOID  (Gr.  juKpo's,  small ;  fwcw,  a  living  thing  ;  eTSos,  form), 
a  small  individual,  in  unicellular  organisms  in  which  two 
sizes  occur. 

MIOCENE  (Gr.  /ueiW,  less ;  Kau/o'j,  recent),  one  of  the  subdivisions 
of  the  cainozoic  era  of  the  earth's  history. 

MITOSIS  (Gr.  /uu'ros,  thread),  the  changes  undergone  by  the  nucleus 
in  typical  cases  of  nuclear  division,  in  which  various  thread- 
like structures  appear. 

MNEMIC  (Gr.  mw,  memory),  a  term  applied  to  a  theory  which 
attributes  the  phenomena  of  heredity  to  a  kind  of  memory. 

MONADS  (Gr.  /uorck,  unit),  minute  unicellular  organisms  of  the 
group  Flagellata. 

MONOECIOUS  (Gr.  /xoVos,  single ;  OLKLOV,  dwelling  place),  having  the 
male  and  female  sexual  organs  in  one  and  the  same 
individual ;  hermaphrodite. 

MONOHYBRIDISM  (Gr.  jxo'vo?,  single ;  Lat.  hybrida,  hybrid),  a 
Mendelian  term  for  a  cross  between  organisms  which  differ 
as  regards  one  pair  of  contrasted  characters. 

MONOPODIAL  (Gr.  jmoVos,  single ;  -nous,  foot),  a  type  of  branch- 
ing in  which  the  main  axis  is  continued  after  each  branch  is 
given  off. 

MONOSOME  (Gr.  judges,  single  ;  o-w/ia,  body),  the  unpaired  chromo- 
some which  appears  in  the  nuclei  of  certain  cells. 


xxx  GLOSSAKY  OF   TECHNICAL   TEEMS 


MONOTREME  (Gr.  juoVos,  single  ;  rp^a,  aperture),  a  member  of  the 

primitive  mammalian  group  Monotren&ta. 
MULTICELLULAR  (Lat.  mnlti,   many  ;    cellula,  cell),  composed   of 

many  cells. 
MUTATION  (Lat.  mutatio,  change),  a  suddenly  appearing  sport  or 

variety,  which  breeds  true. 
NECTARY  (Gr.  vUrap,  the  drink  of  the  gods),  an  organ  in  which 

honey  is  produced  in  a  flower. 
NEURON  (Gr.  vtvpov,  string,  nerve),  a  nerve  cell  with  its  attached 

fibre,  a  unit  of  the  nervous  system. 
NOTOCHORD  (Gr.  i-wrov,  back  ;  x°P^>  strmg)>  a  cord  of  cells  around 

which  the  backbone  is  formed  in  vertebrate  animals. 
NUCELLUS  (Lat.  nucleus,  kernel),  an  inner  part  of  the  ovule  or 

immature  seed,  really  a  macrosporangium. 
NUCLEAR     MEMBRANE,     the     membrane     which     encloses     the 

nucleus. 

NUCLEINIC  ACID,  one  of  the  chemical  constituents  of  the  nucleus. 
NUCLEOLUS  (Lat.  dim.  of  nucleus,  kernel),  a  special  aggregation  of 

chromatin  or  other  substance  in  the  resting  nucleus. 
NUCLEOPLASM    (Lat.    nucleus,   kernel  ;     Gr.    TrAao-jua,   formative 

material),  the  protoplasm  of  the  nucleus. 
NUCLEUS  (Lat.,  kernel),  a  specialised  protoplasmic  body  found 

in  the  interior  of  every  typical  cell. 
ONTOGENY  (Gr.  ovt  oWoj,  a  being  ;  rt.  ytv-,  birth),  the  life-history 

(development)  of  an  individual  organism. 
OOCYTE  (Gr.   o>oV,  egg;  KVTOS,  cell),  an  immature  ovum  ;  a  cell 

from  which  an  ovum  originates. 
OOGENESIS  (Gr.  cooV,  egg  ;  yeWo-is,  origin),  the  process  by  which 

ova  are  formed  in  the  animal  K.dy. 
OOGONIA^  (Gr.  woV,  egg  ;  yoVos,  fruit),  the  female  organs,  containing 

the  oospheres,  in  Fucus,  &c. 
OOGONIA  (Gr.  woV,  egg  ;  yorevj,  ancestor),  the  cells  from  which  the 

oocytes  arise  in  oogenesis. 
OOSPHERE   (Gr.  woV,  egg  ;    a-^alpa,  sphere),  a   name   frequently 

applied  to  the  plant  ovum. 
OPHIUROID  (Gr.  otyus,  snake  ;   ovpa,  tail),  a  brittle-star  or  sand- 

star. 
OPPOSABLE  (Lat.  opponere,  to  place  opposite),  a  term  applied  to 

the  thumb  or  great  toe  when  capable  of  being  placed  with  its 

tip  opposite  to  those  of  the  other  digits. 
ORDOVICIAN,  one  of  the  palaeozoic  epochs  of  the  earth's  history. 


GLOSSARY  OF   TECHNICAL   TERMS  xxxi 

ORGAN  (Lat.  organum,  implement),  a  specialised  part  of  a  living 

body  concerned  in  some  particular  function. 
ORGANELL^E  (Lat.  dim.  organum,  implement),  a  term  sometimes 

applied  to  the  organs  of  unicellular  organisms. 
ORGANISM  (Lat.  organum,  implement),  that  which  has  organs,  viz., 

a  living  body. 
OSMOSIS  (Gr.  wo-juo?,  pressure),  the  diffusion  of  dissolved  substances 

through  semi-permeable  membranes  (upon  which  they  exert 

a  pressure). 
OVARY  (Lat.  ovum,  egg),  in  animals  the  female  gonad,  the  organ  in 

which  eggs  are  produced ;  in  higher  plants  the  part  of  the 

flower  in  which  the  immature  seeds  (ovules)  are  produced. 
OVIDUCT  (Lat.   ovum,  egg ;    ductus,   conduit),  the  tube   or  duct 

through  which  the  eggs  pass  to  the  exterior  of  the  body. 
OVOTESTIS   (Lat.   ovum,   egg ;    testis,   testicle),    a    gonad    which 

produces  both  male  and  female  gametes. 

OVULE  (Lat.  dim.o#Mw,egg),the  immature  seed  of  a  flowering  plant. 
OVUM  (Lat.,  egg),  a  female  germ  cell  or  gamete. 
P^DOGENESIS   (Gr.    irais,   child ;    yeWo-i?,    birth),   the    partheno- 

genetic  production  of  offspring  by  immature  females. 
PALAEONTOLOGY   (Gr.    TtaXaios,  ancient;    ov,   OVTOS,   being;   Xoyos, 

discourse),  the  science  of  fossil  organisms. 
PALAEOZOIC  (Gr.  ira\ai6s,  ancient;    for/,  life),  the  name  given  to 

one  of  the  principal  periods  of  the  earth's  history,  in  which 

ancient  forms  of  life  existed. 
PALINGENETIC  (Gr.  -na\iv,  backwards  [should  be  TraAeuos,  ancient] ; 

yev€(ris,  origin),  of  ancient  origin. 
PANGENESIS  (Gr.  -nav,  all,  every;  ytvea-is,  origin),  the  name  given 

by  Charles  Darwin  to  hL  theory  of  heredity,  in  accordance 

with  which  every  cell  of  the   body   contributes   something 

towards  the  formation  of  the  germ  cells. 

PARAPHYSES  (Gr.  iiapafyvvis,  that  which   grows  alongside),  hair- 
like  structures  which  grow  amongst  the  reproductive  organs 

(in  Fucus,  &c.). 
PARASITE   (Gr.  Trapcuriros,  one  who  lives  at  another's  table),  an 

organism  which  nourishes -itself  at  the  expense  of  another 

living  organism  without  making  any  return. 
PARENCHYMA  (Gr.  Ttap€<yyvv<ai  filling  material),  a  kind  of  ground 

tissue  in  which  various  organs  are  imbedded. 
PARIETAL  (Lat.  paries,  wall),  one  of  the  bones  of  the  skull,  hence  a 

region  of  the  head. 


GLOSSARY  OF  TECHNICAL  TKII.MS 

KHI      (Ol      irap04VQ9t    IM.M'|<H.    ,  .'/'f  <rty,  birMi  >,   I  lie  |M  ,, 


dttotioti  "t  "it  i"  Ing  from  uni.  (  \t\\  ,-.,\ 
Pi  i   -i'    Gh    rlAayofj  MA),  Ih  lag  m  MM-  opwi 

IVuTAfilllNOlh  ((Jr.  W/'Tf,  I'lVO  J  K/u/'or,  lily  ;  |Z$0y,  form),  rftHliin  Ml  nj; 
ii.  I'-  nl  "i  HIM     '    •  .1  M     i.  ii.  l.«  i  MI  upplmd    In    Mm    r.UlKi-d  In.rva 

of  the  f  outlier-Hi  •• 

1,    (Gr,   Wm,    fivo  ;    totarvAo*,   finger),    having   fivn 
or  LOIIH. 

I'l  -.III  III.  KM    (Or.   vif/./,   M.ioiiii'1  .    fl\fj(JMt   rovor),    0110  Of    the    jM-MM-l|.;i.l 

"II  ln.y<  i  ,  111   Mm  umbryoM  of  higher  plantH,  cov«rin«  MM- 


l'i.  in  .\n.  (fli  /ir/u,  IU-OIIIM!  ,  ,ici/;ff  lloHli),  a  horny  proioctive 
ln.yrr  whirl,  iinoiind  M>.  I,.M|V  n,  rnriniii  liydroiM,",  (<•.<,., 
Obilli) 

I'MUIHHOIIM  i  .  i    "ii     /,././..,,.;,,  .,,|,|  ;  ^iK7i'/\n\t  lin^or),  liaviiif<  iui 

M(|<|    llllMil>i    I     nl     lilirM-K!    .,1     |.i  H  :   . 

PlDlH  ii  i  (Oft,  »«/.».  u/AAw,  I  C'.M  hi.i,.  Min  tiiovoinoiil.  <>!'  l.lio 
iilimriilju  y  ciiihil  l.y  innaiiH  <>l  wlm-li  food  in  paHHod  along. 

I'M!  I    I  "Ml  .(      \l     M  il         n^.linrtu-...         I   I.    I.    I  MM  I      MV«    I    ),     MlC     Mil  II     1  1  1  1    I  II  I  .  I  J  |,  |  |  (  1 

Wllloll   lilH'M  t.lio  limly  ciiAily   tnid   r.»v«  •».:   MM-   \i    «••  m,. 
PwilMIAN,    Mi<'    liilr;',!,    of     Mm    p)iJiiM»/,i)ic    r.p«H-|ij:,    Mir    rlnmi,rl,nn;  I  M- 
rnr.lui  nl    \\lin-li.iM    .     l,nni(i  vnly  (Involopod    in    Mm    province   nf 

r<  i  in,  in  i;.M.  ...i.i. 

PlTAU    MJi      ;<«'/,  fA.  ./-,  Inuf),    Mm    nindilir.l    I..,A,.;:    \vliidi    lunn    Mm 

•  'iiolln.  nl  ii,   llnwor. 

|'||A«.",    ,11          Mil        •j'.iyxr.     l.ti     di'Vimi     .       ii'in  -..     rr||),     ;  I  1  1  1  <  i   I  in  1.  1     cell;; 

wlii'-li    'I.  -vniir  nilmr    c«'llii   (»-.»/.,    lnirl,«-i  ui  )    in     Mir    )HII|M-M    nf 
iniill,M'<  llnl.ii   .MIIIIIII.IH. 

|'||A«i"<    ,  M...I  ...    Mm   .M-I.IMII    ul    pliii.i'.iM-yl.rM. 

l'llAI,AN<ii    .  ((  Ii     «/,.iAi.//t    plni.lii.iix  ),  Mm    hniirn    nf    MIC    rm,''rr;i    juid 

I.OOH. 
I'lll.nKM  ((  !i     I/.A..I,,,  .  I  in  ,  :|  »  .  I  lie  on  hi    pii.rf.nl'    Mm  VM.Mcilliir  hi  I  IK  IN1;:, 

I-.  i  nun,"   Mm  hiuil,  (  in   ln;-liri    pliiiil.H). 
I'lin'l'nMYN'i'iii'iniM     ((Jr.     «/"„-.,     h,".hl  .     tii'<i'0tni\,    CI>MI|M  •:  ,il  mn  ),    Mm 

prnrr;,,!    l.y    \\lnrli    cuiiipl*         .  I  .......  .il    n  mi  pi  Ml  nd;  i  ill  r    lunll    lip 

1  1  MI  1  1      i  in  |  .1.  i    i  .,ii    I  il  ih  nl       iindi  i    I  hr   i  n  II  i  n  -i  icr  n|    ,  ;niili;;lil  .  in 
••I-  «  M   plnnl 

'PlIVhliODIO  «ii  r/uJAAor,  lr.il  .  ruutv,  I'm  ml,  n  Irnf  ;:|,-ilk  inndilird  HO 
M,H  i"  roMiuiililn  a  I*  .il 

PlIYI.i'M    ((11.  ./.i'A«»»',    Irnnp.    l.rihr).  .>n<      nf    Mm    priliripn.l    divisinliH 

•  •I    I  Im  iiiiliu.'il    Ki 


<;i,oss.\in    01     nviixicAi,  TI',I;MS         uxiil 


pm  i  001  x^   »(  'i     v;  Vi';  •  h  '''''•  '"  '          ••••-.  "'  ';>m  ).    lll('   :ni1  «  '  '  '  •'' 

1  1  i-.|,.  l  \    .-I    :i    l  :  !«•«'. 
r,\,    M,    (jl.ANIl    "i      l"'.\     ili!  ..     In    .  OIK    ».   .1    l"'d\    Mllil.  li.'d    |,i 

ih,.  up|.,  i     MI  i,i.  M   "i    ih»'    ImniMii    hruin,    i«  •p'^MnliMn    an 

ilililll  K'll;ll     p:ill     .'I    <    \,          |'K       .    Ill      III     MIKi<     il:ll     \  01  !<<h|  MltVi 

)  'j-,  |  1  1,    (|,:»l.   /»/.••///////><,    pOHMo),  M   CtdliM'l  i\<»   I  ci  in    !«  >i     I  1  1<     "  It  ni:(|(> 

01'tfMIKt  of  IL  llowur,   =  tf.Yli..  ,  mm 
PlTUITAKN     non\    tl  -Ml.    fiitiul,!.    •.Inn.-,   inihii    >.  M    |*lan<liiliir   hotly 

;ill:i,   In  ,1    In    lll<-    Illlili   I        III  I:H  (\   "I'   l,ll(>    lu'lllll 
|'l,\,|,\i\    (l,il    .    a    ll.il    «!ilv<>).    |,ho    <>|>',llll    h\    \\lll.h.    ill    lii:iinni:il    . 

III,.    firiiiM    i      iilliit  hi'.l   in    lh«<  \\inuh  Mild   Mir(Hl|;h   \\ln.  h    n    i 

ii«iiiri;«li«  -I 
Pl,ANM«'N    ((il'-    irAciyKt..c.    \\.in.l.-i  in;;),    (lie    llmil  ill;',    populM  I  i.*n    QJ 

M'MS    Mild    Li  I.  ' 

I'l  \    M  \   i(.i       iAii<r/jiu,    I'm  ni.ii  i\  <>   iiiMl(MiMl).    Ilic   h<|iiid    portion    "I 


I'l  y  M,.I>I  \  ((  ii     ,,  A,  •../ 1...  I.  M  HIM!  i\ «    1 1  in  I.  M  nil  i.  mull  mm  ICM!.-  n 

Of      p|..|,.|i|;|    .III.     I'M   Ml.   <|       |,\      |  |  ),.     HI  I  I.  HI     M|       IIIMII\      Mill..    I  M  M.I      .    .    II 

<  in    \l\ •  •«  i"   08  ' 

Mi,     In  .I.HI  i.i  HIP  rylnphiMlil   «d    I  \\ . .  .  •  1 1 

I'l    \      l  I.    I  I  \      l<  ,  I  IIHII||I|IM|  ).     I  hi'     |K'\M'I      «>|      I  <  "   | '«•  1 1  d  I  1 1 ' '     I" 

.  li.ili"r •;    in    I  ll<<   i   li\  II  .'lilil.lil. 
Pl.x-.i  M.',    ((  ii  tdrl  ;  <•"..    i Ii  Ili-i  I.    pi»h<iil     I'udi.  :;    \\  In.  Ii    urnn     in 

I  In    cyl  Oplai I   .  .  i  l.iin  .  .  II  .  Mild   !IM\  •    MM     powiH  of  fc 

•    |H    ,    l.ll         III.     I.M.IK    . 

I'l.  \  '.I  MI;  \  M\  .    Hf>,< 

PLIXITOOINK  (Or.  nA»i noil  l   '•"•—    «..  nh.  MM   i.itmt  of 

I  1 1  ( *  .  • :  i  n  i .  i , . .  1 1 .  •   .  |  M  ' .  1 1 

PU<;I!MMI';  (Oi    n  M/. i'".  i  I'.'i  whii  Ii  nil:;  upi.  (In   CM  niral  .  \  lmd<  i  t)i 

l,  i,  ni  ntti  d  . .  Ih;  111  ih«  I'liihryo  .-I  MM    In'  I"  i   pi. i 
]  (Or,   TrAc^oDi*,  more;   KOU^V,  reormi),  on«'  *d    MM     ni 

dl\  l    ioil      .'I    MM     «  IIJI • I    MM     .  ,n  I  I,        In    I, .IN 

I  'l    i    M   i  (hill,.,      I"   I'l     hoilMl    i      I  IK        II  «  i         'A  iMiliilli"       lii  \  .1      i.| 

ill  .Inn      .uid    hi  ll  Mi      ltd  I 

I'm, \i;    i  om  .   .1 1"  rfoi'l    •  •  II     •  I'M  ah  .1    from   nnn   po|c«  nf    1 1.. 

M\  inn   dill  in;-    HIM!  in  .il  mil. 
Pnl.l.r/.    •  I  ifct  .  Il du    i  i.    .     I.IK     dttli    '  Oni  II  I  [fi|    "I    I  h«     nn.  ffl 

i "i  polloii  ^I'Min  "i  no  wiring  planl 

I'..  I   I   l         l  I..',.    I  IK      ,i  |.|.ll.  ,il "I     pi  •  Hi   n     In     I  h<        I  i"  i  M.I    i.|     ||    I  |,i  \\  rr 

('•II'   H    I I"         polti   n    "'I    -i       I"    l  I  ill     :il  l 

n.  r 


xxxiv          GLOSSAKY  OF   TECHNICAL   TEEMS 

POLLINIA,  solid  aggregations  of  pollen  grains  found  in  the  flowers 

of  orchids. 
POLYDACTYLISM   (Gr.    TToXvbaKTvXos,   many-toed),   a   condition    in 

which  more  than  the  normal  number  of  fingers  or  toes  are 

present. 
POLYMOEPHIC   (Gr.    77oAt^iop<£o9,  multiform),  occurring    in  many 

forms. 
POLYPE  (Gr.  -noX.vTrov$,  many  footed),  a  name  applied  to  Hydra 

and  various  other  Coelenterate  animals   in   allusion  to  the 

numerous  tentacles. 
POSTAXIAL  (Lat.  post,  behind;  axis),  lying  behind  the  axis  of  a 

limb. 
PEEAXIAL  (Lat.  prce,  in  front ;  axis),  lying  in  front  of  the  axis 

of  a  limb. 
PRE -FORMATION,  an  old  theory  in  accordance  with  which  every 

egg   contains   a  complete  miniature  of  the   organism  into 

which  it  will  develop. 

PREHENSION  (Lat.  prehendere,  to  seize),  catching  hold. 
PRIMORDIA    (Lat.  primordium,  origin),  hypothetical  particles  of 

the  germ  plasm  which  are  supposed  to  be  responsible  for 

the  development  of  special  inheritable  characters. 
PRIMORDIAL   UTRICLE  (Lat.  primordium,  origin  ;    utriculus,  little 

bag),  the  protoplasmic  lining  of  a  typical  vegetable  cell,  next 

to  the  cell  wall. 
PROBOSCIS  (Gr.  TT/JO/SOO-KIS,  trunk,  snout),  a  term  somewhat  loosely 

applied  to  any  prehensile,  suctorial  or  other  organ  which 

projects  from  the  neighbourhood  of  the  mouth. 
PROCRYPTIC  (Gr.  -npo,  on  behalf  of  ;  KpuTirds,  hidden),  a  term  applied 

to  protective  resemblance  whereby  concealment  is  effected. 
PRONUCLEUS  (Lat.  pro,  in  place  of >,  nucleus,  kernel),  the  reduced 

nucleus  of  a  mature  gamete  before  it  unites  with  its  mate 

to  form  the  zygote  nucleus. 
PROTANDROUS  (Gr.  vp^-os,  first ;  avfip,  man,  male),  a  term  applied 

to   hermaphrodite   organisms   in   which    the   male   organs 

mature  before  the  female. 
PROTEIDS,  PROTEINS  (Gr.  npwrtvs,  the  changeable  one),  a  group  of 

complex   chemical  compounds  which  form  the  chief  con- 
stituents of  protoplasm. 
PROTHALLUS  (Lat.  pro,  in  place  of ;  thallus,  a  green  stalk),  the 

sexual  individual  (gametophyte)  in  ferns,  &c. 
PROTOGYNOUS  (Gr.  -np&rov,    first;  yvvri,  woman,  female),   a  term 


GLOSSARY   OF   TECHNICAL   TERMS  xxxv 

applied  to  hermaphrodite  organisms  in  which  the  female 

organs  mature  before  the  male. 
PROTOPLASM  (Gr.  irp&Tos,  first ;  7rAao>ta,  formative  material),  the 

essential   constituent  of   all  living  bodies  ;    "  the  physical 

basis  of  life." 
PROTOZOON    (Gr.    -p^ros,    first  ;     Cyov,    animal),    a    unicellular 

animal. 
PSEUDOPODIUM(GI*.  \lf€vbi]s,  false;  TTOVS,  foot), a  temporary,  irregular 

protoplasmic  projection  from  a  naked  cell,  generally  used  as 

an  organ  of  locomotion  (e.g.,  in  Amoeba). 
PYRENOID  (Gr.  itvpriv,  fruit  stone ;  ei5os,  form),  a  starch-forming 

proteid  body,  found  in  certain  cells. 
RADIUS  (Lat.,  spoke),  one  of  the  bones  of  the  forearm. 
RECAPITULATION  HYPOTHESIS,  the  theory  in  accordance  with  which 

an  individual  organism  repeats  its  ancestral  history  in  its 

own  development. 
RECEPTOR  (Lat.,  receiver),  an  organ  which  receives  stimuli,  a  sense 

organ. 
REGRESSION,  FILIAL  (Lat.  regressus,  a  going  back),  a  return  on 

the  part  of  the  offspring  of  exceptional  parents  towards  the 

average  condition  of  the  species. 
RESPIRATION  (Lat.  respiratio,  breathing),  essentially  the  interchange 

of  the  two  gases,  oxygen  and  carbon  dioxide,  the  former  being 

taken  in  by  the  organism  (or  tissue)  and  the  latter  given  out. 
REVERSION  (L&t.reversio,  a  turning  back),  the  sudden  reappearance 

of  some  long-lost  ancestral  character. 
RODENTS  (Lat.  rodere,  to  gnaw),  animals  of   the  order  Rodentia 

(e.g.,  rabbits,  mice). 
ROSTELLUM  (Lat.  dim.  rostrum,   beak,  snout),  one  of  the  three 

stigmas  of  an  orchid  modified  to  form  a  kind  of  cement  gland. 
ROTIFERS  (Lat.  rota,  wheel ;  ferre,  to  carry),  wheel  animalcules, 

a  group  of  minute  aquatic  worms  with  cilia  arranged  like 

wheels. 
SAPROPHYTIC  (Gr.  (Tempos,  putrid ;  AUTO'S,  growing),  living  on  dead 

organic  matter. 

SARCODE  (Gr.  <rdp£,  flesh ;  et5os,  form),  an  old  term  for  proto- 
plasm. 

SCAPULA  (Lat.,  shoulder  blade),  the  shoulder  blade. 
SECRETION  (Lat.  secretus,  separate),  a  useful  substance  separated 

or  secreted  by  some  special  organ  (gland) ;  or  the  process 

involved  therein. 


xxxvi  GLOSSARY  OF   TECHNICAL   TERMS 

SEDIMENTABY  (Lat.  sedere,  to  settle  down),  a  term  applied  to  rocks 

deposited  as  sediment  under  water. 
SEGMENTATION  (Lat.  segmentum,  a  cutting),  division  into  parts,  as 

in  a  dividing  ovum  or  in  the  segmented  body  of  a  worm. 
SEGEEGATION  (Lat.  segregare,  to  set  apart),  separating  out. 
SEMATIC  (Gr.  o-rj/xa,  signal),  a  term  applied  to  colours  used  as 

signals. 
SEPALS  (Lat.  sepes,  fence,  enclosure),  the  modified  leaves  which 

form  the  outermost  whorl  (calyx)  of  a  typical  flower. 
SILUEIAN   (Lat.  Silures,  an   ancient  British  tribe),   one  of  the 

palaeozoic    epochs,    the    characteristic   rocks   of  which    are 

largely  developed  in  Wales,  &c.  (Siluria). 
SOMA  (Gr.  o-w/xa,  body),  the  body  of  an  organism,  contrasted  with 

the  germ  cells. 

SOMATIC,  pertaining  to  the  soma  or  body. 
SOMATOGENIC  (Gr.  <rw/xa,  body ;   rt.  ytv-,  origin),  originating  in  the 

soma  or  body. 
SOMATOPLEUEE  (Gr.  crw/xa,  body ;   vXtvpa,  side),  the  body  wall, 

enclosing  the  body  cavity. 
SOMITE  (Gr.  o-co/xa,  body),  one  of  the  divisions  of  which  a  meta- 

merically  segmented  body  is  made  up,  =  metamere. 
SPECIES  (Lat.,  sort,  kind),  a  group  of  individuals  which  closely 

resemble   one   another   owing    to    descent    from    common 

ancestors  and  at  the  same  time  differ  from  all  other  species 

(see  p.  223). 
SPECIFIC    CHAEACTEES,    characters    by    which     one     species    is 

distinguished  from  others. 
SPEEMAEY  (Gr.  (r-n-e'p/xa,  seed,  sperm),  a  general  term  for  a  gonad 

in  which  male  gametes  are  produced  (testis). 
SPEEMATID  (Gr.  a-neppa,  sperm),  a  cell  which  develops  directly 

into  a  spermatozoon. 
SPEEMATIST  (Gr.  a-nepua,  sperm),  an  old  term  applied  to  one  who 

believed  that  the  animal  embryo  was  produced  entirely  by 

the  male. 
SPEEMATOCYTE  (Gr.  o-Tre'p/xa,  sperm ;  KVTOS,  cell),  a  cell  from  which 

spermatids,  and  hence  spermatozoa,  originate. 
SPEEMATOGENESIS  (Gr.  mrep/xo,  sperm ;  yevecns,  origin),  the  process 

by  which  male  gametes  (spermatozoa)  originate. 
SPEEMATOGONIA  (Gr.   o-n-e'p/xa,   sperm;  yovevs,  ancestor),  the  cells 

from  which  the  spermatocytes  originate  in  spermatogenesis. 
SPEEMATOZOID,  a  term  sometimes  applied  to  a  plant  spermatozoon. 


GLOSSARY  OF   TECHNICAL   TERMS         xxxvii 

SPERMATOZOON  (Gr.  o-Trep/xa,  sperm  ;  Cyov,  a  living  being),  the 
term  applied  to  a  male  gamete  or  germ  cell  when  differen- 
tiated as  a  free-swimming,  active  organism. 

SPICULES  (Lat.  spiculum,  a  little  sharp  point),  minute  mineral 
bodies,  usually  of  definite  shape,  secreted  by  special  cells  and 
forming  the  skeleton  of  many  invertebrate  animals  (e.g., 
certain  sponges). 

SPIREME  (Gr.  a-ntLpwa,  coil),  the  coiled  thread  of  chromatin  sub- 
stance which  occurs  at  a  certain  stage  in  mitosis. 

SPLANCHNOPLEUEE  (Gr.  o-n-Aayx^a,  viscera;  irXevpd,  side),  the  gut 
wall,  as  opposed  to  the  body  wall. 

SPONTANEOUS  GENERATION,  the  (supposed)  origin  of  living  things 
from  not-living  matter. 

SPORANGIUM  (Gr.  a-iropd,  seed ;  ayyetor,  vessel),  a  capsule  in 
which  spores  are  produced. 

SPORE  (Gr.  a-iropd,  seed),  a  single  cell  which  is  capable  of  giving 
rise  without  any  sexual  process  to  a  new  individual  (e.g.,  in 
ferns)  ;  absolutely  different  from  a  true  seed. 

SPOROPHYLL  (Gr.  cnropd,  seed ;  (f>v\\ov,  leaf),  a  spore-bearing 
leaf. 

SPOROPHYTE  (Gr.  a--opd,  seed ;  fyvrov,  plant),  the  asexual,  spore- 
producing  generation  in  certain  plants  (e.g.,  ferns  and 
flowering  plants). 

SPORT,  a  suddenly  appearing  variation  ;  a  mutation. 

STAMENS  (Lat.  stamen,  thread),  the  modified  leaves  which  form 
the  "  male  "  organs  of  a  flower. 

STAMINODES  (Lat.  stamen,  thread ;  Gr.  eTSoj,  form),  imperfect 
stamens. 

STATOBLAST  (Gr.  araros,  standing ;  jSAaoros,  bud),  a  dormant  bud, 
protected  by  a  special  envelope ;  found  in  certain  fresh-water 
invertebrates. 

STIGMA  (Gr.  o-rty/ua,  spot),  the  moist  spot  on  the  end  of  the  style 
of  a  flower,  on  which  pollen  is  deposited. 

STOMA  (Gr.  orojua,  mouth),  a  minute  opening  in  the  epidermis  of 
a  leaf,  leading  into  air  spaces  in  the  mesophyll. 

STRATIFIED  (Lat.  stratum,  pavement),  arranged  in  layers. 

STYLE  (Gr.  or{5Ao9,  pillar),  part  of  the  "  female "  organs  of  a 
flower,  bearing  the  stigma. 

SUPRASCAPULA  (Lat.  supra,  above  ;  scapula,  shoulder  blade),  a  bone 
or  cartilage  which  lies  above  the  scapula  in  the  shoulder 
girdle. 


xxxviii          GLOSSARY   OF   TECHNICAL   TERMS 

SUSPENSOR  (Lat.  suspend  ere,  to  hang  up),  a  row  of  cells  by  which 

/the  embryo  is  attached  to  the  embryo  sac  in  flowering  plants. 
MATIC  (Gr.  cruv,  together ;  dn-o,  away  from  ;  o-rj^a,  signal), 

sharing  a  common  warning  colour. 
SYNAPSIS  (Gr.  vvvatyis,  conjunction),  the  pairing  of  chromosomes 

prior  to  nuclear  reduction. 
SYNCYTIUM  (Gr.  <rvv,  together :  KVTOS,  cell),  a  layer  of  protoplasm 

not  divided  into  separate  cells  but  with  many  nuclei. 
SYNDESIS   (Gr.   (rvvbrjaai,   to    bind    together),   another   term   for 

synapsis  (q.v.). 
SYNGAMY  (Gr.  crvv,  together ;  ydjuos,  marriage),  the  sexual  union 

of  gametes  (conjugation). 

TARSALS  (Gr.  rapo-os,  foot  [ankle]),  the  bones  of  the  ankle. 
TAXONOMY  (Gr.  raft?,  arrangement ;   vopos,  law),  the  science  of 

classification. 

TESTIS  (Lat.,  testicle),  an  organ  in  which  male  gametes  are  pro- 
duced ;  a  male  gonad. 
THYLACINE    (Gr.    QuXaKos,    pouch  ;    KVWV,    dog),   a    carnivorous 

marsupial  (pouched  mammal)  found  in  Tasmania. 
TIBIA  (Lat.,  shin-bone),  one  of  the  bones  of  the  leg. 
TORPEDO   (Lat.,  numbness,  electric   ray),  a   kind  of  ray,    with 

powerful  electric  organs. 
TRIASSIC  (Gr.  rpids,  three),  the  earliest  epoch  of  the  mesozoic  era, 

so-called  from  the  three-fold  subdivision  seen  in  Germany. 
TRICHOCYSTS   (Gr.    0/oi'f,  hair;    KVO-TIS,  pouch),  minute   sacs  con- 

taining  hair-like  processes  (in  Paramoecium,  &c.). 
TRILOBITE  (Gr.  r/oety,  three ;  Ao/3o's,  lobe),  an  extinct  crustacean 

of  the  paleozoic  era. 
TURBELLARIAN  (Lat.  turbella,  stir),  a  kind  of   flat  worm  whose 

ciliated  surface  stirs  up  the  water. 
ULNA  (Lat.,  arm),  one  of  the  bones  of  the  forearm. 
UNGULATE  (Lat.  ungula,  hoof),  a  hoof -bearing  animal  of  the  order 

Ungulata. 

UNGULIGRADE  (Lat.  ungula,  hoof ;  gradus,  step),  walking  on  the  hoof. 
UNICELLULAR  (Lat.  unus,  one ;  cellula,  cell),  consisting  of  a  single 

cell. 
UNISEXUAL  (Lat.  unus,  one ;  sexus,  sex),  either  male  or  female,  not 

hermaphrodite. 
UREA  (Gr.  ovpov,  urine),  a  nitrogenous  compound  formed  as  a 

waste  product  in  animal  bodies. 
UTERUS   (Lat.,  womb),  an  enlarged  portion  of   the    oviduct,  in 


GLOSSARY  OF   TECHNICAL   TERMS  xxxix 

which  an  embryo  or  foetus  undergoes  development ;  the  womb 

in  mammals. 
VACUOLB  (Lat.  vacuus,  empty),  a  space  in  the  protoplasm  of  a  cell, 

filled  with  gas  or  liquid. 
YAS  DEFERENS  (Lat.,  a  vessel  which  carries  down),  a  duct  through 

which  spermatozoa  pass  to  the  outside  of  the  body. 
VASCULAR  BUNDLE  (Lat.  vasculum,  a   small  vessel),  a  bundle  of 

vessels,  &c.,   through   which   sap  is   distributed   in  higher 

plants. 
VERTEBRA  (Lat.,  joint,  vertebra),  one  of  the  segments   of  the 

backbone. 

VERTEBRATE,  having  a  backbone. 
VESTIGIAL  (Lat.  vestigium,  trace,  vestige),  reduced  almost  to  the 

point  of  disappearance. 
VITALISM   (Lat.   vita,   life),   the   doctrine^jg^ch    attributes   the 

phenomena  of  life  to  a  special  vitaTforce.* 
VITELLINE  MEMBRANE  (Lat.  vitellus,  yolk),  a  delicate  membrane 

which  encloses  an  ovum. 

XYLEM  (Gr.  £vXov,  wood),  the  woody  part  of  a  vascular  bundle. 
ZO^EA   (Gr.    C*?0^   animal),  a   larval   stage   of  crabs  and  other 

Crustacea. 
ZONA  RADIATA  (Lat.,  radiate  zone),  a  membrane  which  encloses  the 

ovum  in  mammals,  outside  the  vitelline  membrane. 
ZOOID  (Gr.  6jW,  animal  ;  €1609,    form),   an   individual   member 

of  a  colony  (e.g.,  in  Obelia). 
ZOOPHYTE  (Gr.  fwor,  animal ;  fyvrov,  plant),  an  old  term  applied  to 

certain  plant-like  invertebrates  (e.g.,  Obelia). 
ZOOSPORE  (Gr.  fooo's,  alive  ;  a-nopd,  seed),  an  actively  swimming 

spore. 

ZYGOSIS  (Gr.  Qyavis,  joining   together),  another  term  for  con- 
jugation or  the  sexual  union  of  gametes. 
ZYGOSPORE  (Gr.  C^yo'co,  I  join  together  ;    a-iropd,  seed),  a  zygote 

formed  by  the  conjugation  of  two  similar  gametes  (e.g.,  in 

Spirogyra). 

ZYGOTE  (Gr.  (bywros,  joined  together),  a  cell  produced  by  the  con- 
jugation of  two  gametes  and  capable  of  developing  into  a  new 

individual. 


ORGANISMS  AND  MACHINES  3 

cation  to  its  environment,  and  whose  action  consists  in 
?ontinual  self -adjustment  to  changes  in  that  environment. 
What  we  call  the  life  of  the  organism  consists  of  the  sum  total 
of  all  the  activities  which  it  thus  exhibits.  The  question  at  once 
arises :  How,  then,  does  a  living  organism  differ  from  a  mere 
man-made  machine  ?  and  this  question  is  one  which  it  is  by  no 
means  easy  to  answer.  An  organism,  however,  is  not  merely  a 
piece  of  apparatus  which  has  the  power  of  maintaining  itself  for 
a  longer  or  shorter  period  in  a  state  of  equilibrium  with  its 
environment  and  thereby  preserving  itself  from  destruction,  for 
it  also  has  the  power  of  reproducing  its  kind  by  a  process  of  self- 
multiplication.  In  the  case  of  an  artificial  machine,  where  there 
is  little  or  no  automatic  adjustment ,  the  forces  of  the  environ- 
ment very  soon  get  the  upper  hand ;  the  metal  work  becomes 
corroded  by  oxidation,  or  worn  away  by  friction,  and  presently 
the  whole  affair  comes  to  a  standstill.  Oxidation  and  friction, 
and  innumerable  other  chemical  and  physical  agencies  also  tend 
to  destroy  the  machinery  of  the  living  body,  but  for  a  longer  or 
shorter  period  thev  are  held  in  check  by  automatic  processes  of 
repair  and  renewal,  and  when  the  inevitable  end  does  come  it  is 
usually  not  until  the  organism  has  produced  at  least  enough  off- 
spring to  take  its  place  in  the  struggle  for  existence. 

One  of  the  most  brilliant  writers  of  the  nineteenth  century, 
Samuel  Butler,  has  indulged  in  the  somewhat  fantastic  sugges- 
tion that  some  day  the  construction  of  machines  might  be  so 
perfected  that  they  also  would  be  able  to  reproduce  their  kind,  and 
the  little  steam-engines  would  be  seen  playing  about  the  door  of 
the  engine  shed.  It  certainly  does  not  seem  possible  that 
machines  will  ever  multiply  in  this  way,  but  should  they  do  so, 
and  should  they  at  the  same  time  be  able  to  feed  and  grow,  it  is 
difficult  to  see  why  they  should  not  be  as  much  entitled  to  be 
called  living  organisms  as  any  of  the  plants  and  animals  which 
inhabit  the  earth  to-day.  They  would,  however,  still  be  totally 
different  from  plants  and  animals  both  in  structure  and 
composition. 

One  of  the  most  remarkable  and  characteristic  features  of  the 
living  things  which  inhabit  this  earth  is  that  they  are  all  com-' 
posed  of  very  similar  materials,  whijj^ire  very  different  in  their 
nature  from  any  which  we  should  be  likely  to  choose  in  the 
construction  of  a  machine.  In  making  an  engine  we  select  those 
substances  which  seem  best  calculated  to  resist  the  destructive 

B  2 


4     OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

action  of  the  environment ;  hard  and  rigid  metals  which  will 
bear  heavy  strains,  and  as  far  as  possible  such  as  will  be  proof 
against  the  chemical  action  of  the  atmosphere ;  and  we  do  our 
best  by  means  of  oil  and  paint  to  protect  even  these  from  injurious 
influences. 

A  living  body  may  also  have  its  hard  protective  structures,  as 
the  shell  of  the  oyster  and  the  scales  of  the  fish,  or  its  rigid 
levers,  as  the  bones  in  our  own  limbs,  but  the  really  essential 
part  of  the  organism  is  built  up  of  just  those  materials  which  are 
most  liable  to  destruction  by  chemical  and  physical  agencies — of 
that  almost  liquid  and  extremely  unstable  substance  which  we 
have  already  referred  to  as  protoplasm,  and  of  its  various 
derivatives.  The  experience  of  every  day  teaches  us  how 
rapidly  the  bodies  of  animals  and  plants  decay  when  they  are 
left  exposed  to  the  atmosphere  after  life  has  become  extinct ;  and 
this  decay  is  simply  the  destruction  caused  by  the  disintegrating 
forces  of  the  environment.  A  disabled  steamboat  may  lie 
rusting  on  the  shore  for  many  years  without  undergoing  much 
change,  but  the  dead  body  of  a  stranded  jelly-fish  thrown  up 
beside  it  will  become  disorganized  and  disappear  in  the  course  of 
a  few  hours,  and  yet  the  jelly-fish  when  alive  was  undoubtedly 
the  more  complex  and  perfect  piece  of  apparatus. 

The  body  of  an  organism,  moreover,  undergoes  destruction 
not  only  after  death  ;  it  is  always  undergoing  destruction,  and 
its  very  life  depends  upon  its  destruction,  just  as  the  flame  of  a 
candle  depends  upon  the  destruction  of  the  candle.  But  as  it  is 
destroyed  it  is  constantly  built  up  again;  new  protoplasm  is 
formed  and  new  tissues  take  the  place  of  those  which  are  worn 
out.  The  life  of  the  organism  is,  in  fact,  the  outcome  of  the 
constant  struggle  between  destructive^ and  constructive  forces, 
and  the  keener  the  struggle  the  more  vigorous  will  be  the  life- 
just  as  the  flame  will  be  brighter  or  hotter  in  proportion  to  the 
activity  of  the  combustion  to  which  it  owes  its  existence. 

Life,  like  the  flame,  is  a  manifestation  of  energy,  and  tke 
living  body  is,  like  the  steam-engine,  a  machine  for  transforming 
one  kind  of  energy  into  another.  Moreover,  the  ultimate  source 
of  the  energy  is  the  faame  in  both  cases.  That  of  the  steam- 
engine  is  derived  from  the  combustion  or  oxidation  of  coal,  whioii 
contains  stores  of  energy  derived  millions  of  years  ago  from  the 
light  and  heat  of  the  sun  by  the  green  plants  which  flourished 
in  the  vast  forests  of  the  Carboniferous  epoch.  The  green  plants 


SOURCE   OF   ENERGY   OF   ORGANISMS  5 

of  to-day  still  derive  supplies  of  energy  from  the  same  source 
and  lock  up  in  their  leaves  and  stems  what  they  do  not  them- 
selves expend,  while  the  animals  in  turn  derive  their  energy 
from  the  green  plants  upon  which  they  directly  or  indirectly 
feed.  Hence  both  plants  and  animals  are  ultimately  dependent 
upon  the  sun  for  their  existence. 

Even  the  most  superficial  examination  is  sufficient  to  demon- 
strate the  fact  that  the  body  of  any  of  the  more  familiar  animals 
or  plants  is,  as  we  have  already  indicated,  an  extremely  complicated 
thing.  Whatever  may  be  the  degree  of  complexity,  however,  and 
however  much  one  organism  may  differ  from  another  in  details 
of  structure,  whether  it  be  a  microscopic  alga  or  an  oak  tree,  an 
Amoeba  or  a  man,  there  are  always  certain  things  which  have  to 
be  done,  certain  actions  or  functions  which  have  to  be  performed, 
in  order  that  its  life  may  be  maintained. 

In  the  first  place,  the  organism  must  safeguard  itself  as  far  as 
possible  from  the  destructive  influences  of  its  environment.  It 
must  not  only  be  able  to  protect  itself  from  such  physical  agents 
as  heat  and  cold,  mechanical  impact  and  friction,  but  it  must  be 
able  to  select  a  situation  where  life  is  possible,  and  to  escape 
from  other  organisms  by  which  it  is  liable  to  be  attacked.  All 
this  involves  the  expenditure  of  energy  in  some  form  or  another ; 
it  may  be  in  the  manufacture  or  secretion  of  protective  envelopes 
or  shells,  such  as  we  find  even  in  some  of  the  simplest  Protozoa, 
or  it  may  be  in  actively,  moving  away  from  the  source  of  danger. 
Thus  it  appears  that  the  very  first  thing  necessary  for  the 
maintenance  of  life  is  the  expenditure  of  energy.  This 
energy,  though  ultimately  derived  from  the  sun,  is,  as  we  have 
already  seen,  derived  .  immediately  from  the  combustion  of 
fuel,  very  much  as  in  the  case  of  a  steam-engine,  but  with  the 
important  difference  that  in  the  living  organism  the  fuel  is,  at 
any  rate  to  a  large  extent,  the  actual  substance  of  which  the 
body  is  composed.  In  this  respect  the  comparison  with  a  candle 
is  especially  apt,  for  it  is  the  combustion  of  the  actual  substance 
of  which  the  candle  is  composed  that  liberates  the  energy 
manifested  in  the  light  and  heat  of  the  flame. 

Now  combustion  is,  of  course,  simply  another  name  for  what 
chemists  term  oxidation,  or  combination  with  the  element 
oxygen,  a  process  which  is  often  accompanied  by  the  liberation 
of  a  considerable  amount  of  energy  in  the  form  of  heat  and 
light,  though  this  is  by  no  means  always  the  case.  In  the 


6  OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

higher  animals  sufficient  heat  is  evolved  to  maintain  the 
temperature  of  the  body  at  a  level  considerably  above  that  of  the 
surrounding  atmosphere,  and  such  animals  are  accordingly 
termed  "  warm-blooded  "  ;  in  plants,  on  the  other  hand,  and  in 
the  "  cold-blooded  "  lower  animals,  the  amount  of  heat  evolved 
is  not  as  a  rule  sufficient  to  raise  the  body  temperature  to  any 
great  extent,  if  at  all.  Heat,  however,  is  only  one  form  in  which 
energy  may  be  manifested,  and  in  living  organisms  it  is,  as  a 
matter  of  fact,  much  more  conspicuously  manifested  in  the  form 
of  motion,  especially  in  animals,  while  in  not  a  few  cases  even  a 
low  temperature  combustion  may  liberate  energy  in  the  form  of 
light,  as  in  the  glow-worm  and  numerous  other  luminous 
animals  and  plants. 

When  a  piece  of  charcoal  is  burnt  in  the  air  it  enters  into 
combination  with  the  elementary  oxygen  gas  of  the  atmosphere, 
and  another  invisible  gas  which  we  term  carbon  dioxide,  or 
carbonic  acid,  is  produced,  in  accordance  with  the  equation — 

C  +          02  C02 

(Carbon)  (Oxygen)  (Carbon  Dioxide). 

In  this  process  energy  is  set  free  in  the  form  both  of  heat  and 
light.  We  must  now  inquire  a  little  more  carefully  whence  this 
energy  really  comes,  for  although  this  is  a  question  primarily 
for  the  chemist  and  physicist  it  is  also  clearly  one  which  the 
biologist  cannot  afford  to  leave  unanswered. 

In  accordance  with  the  principles  of  the  conservation  of 
energy  and  the  indestructibility  of  matter  we  believe  that  the 
quantities  of  energy  and  matter  which  exist  in  the  universe 
are  fixed  and  constant.  Neither  energy  nor  matter  can  be 
created  and  neither  can  be  destroyed,  though  each  may  express 
itself  in  a  great  variety  of  ways  and  change  more  or  less  readily 
from  one  mode  of  expression  to  another.  Thus,  as  we  have 
already  seen,  the  energy  of  the  sun's  rays  may  be  utilized  in 
building  up  the  bodies  of  green  plants,  and  locked  up,  as  it  were, 
in  the  substances  of  which  these  are  composed. 

We  also  know  that  different  chemical  elements  have  a  very 
strong  "  affinity "  for  one  another,  their  atoms  tending  to 
unite  and  form  compound  molecules  when  they  are  brought 
within  the  sphere  of  each  other's  attraction.  Once  united 
they  can  only  be  separated  again  by  the  expenditure  of  energy, 
and  when  they  unite  a  corresponding  amount  of  energy  is 


ENEEGY  OF  CHEMICAL  AFFINITY  7 

liberated.  We  may  say,  for  example,  that  in  the  elements 
carbon  and  oxygen,  so  long  as  they  remain  separate,  a  certain 
amount  of  energy  remains  latent.  We  call  this  potential 
energy.  When  the  carbon  and  oxygen  atoms  are  allowed  to 
come  together  and  unite,  this  potential  energy  of  chemical 
affinity  is  liberated  as  kinetic  energy,  and  manifested  in  the 
form  of  light  and  heat. 

It  is  from  the  potential  energy  of  chemical  affinity  that  the 
energy  of  a  living  organism  is  immediately  derived.  Protoplasm, 
the  fundamental  constituent  of  both  plants  and  animals,  contains 
chemical  compounds  of  extremely  complex,  structure,  composed 
of  many  elements  and  containing  a  large  amount  of  potential 
energy  locked  up  in  them.  Moreover,  these  proteids,  as 
they  are  termed,  are  extremely  unstable  bodies,  readily  breaking 
up  on  oxidation  into  simpler  and  more  stable  combinations  and 
thus  liberating  energy.  It  is  the  presence  of  these  unstable 
proteids  which  confers  upon  protoplasm  its  peculiar  fitness  to 
form  what  has  been  so  aptly  termed  by  Huxley  "  The  physical 
basis  of  life."  They  play  the  part  of  the  gunpowder  in  a 
cartridge,  ready  to  produce  a  manifestation  of  energy  as 
soon  as  the  proper  stimulus  is  applied. 

It  is,  then,  the  breaking  up  of  proteids,  or  of  some  other 
complex  substances,  usually  by  recombination  of-  their  con- 
stituents with  oxygen,  which  furnishes  the  constant  supply  of 
energy  which  an  organism  requires.  This  process,  however, 
can  only  go  on  so  long  as  the  supply  of  combustible  matter  on 
the  one  hand  and  of  oxygen  on  the  other  is  adequately  main- 
tained, ajid  this  brings  us  to  the  consideration  of  two  of  the 
most  important  functions  which  every  living  organism  must 
perform,  nutrition  and  respiration. 

Under  the  head  of  nutrition  we  must  include  all  those 
.processes  which  are  concerned  in  building  up  the  body,  in 
making  good  the  waste  of  substance  necessitated  by  the  expendi- 
ture of  energy  and  thus  providing  new  stores  of  fuel  for  the 
use  of  the  organism.  The  first  step  in  nutrition  is  the  taking 
into  the  body  of  suitable  food  material.  In  the  case  of  the 
typical  animal  this  material  must  contain  in  some  form  or 
other  all  the  necessary  supply  of  potential  energy,  locked  up  in 
more  or  less  complex  and  unstable  chemical  compounds  such 
as  it  can  obtain  only  from  the  bodies  of  other  organisms.  The 
green  plant,  on  the  other  hand,  by  virtue  of  the  chlorophyll 


8  OUTLINES   OF  E VOLUTION AKY  BIOLOGY 

which  it  contains,  has  the  power  of  absorbing  energy  directly 
from  the  sun's  rays  and  using  this  to  build  up  the  complex 
proteids  from  very  simple  constituents.  The  feeding  of  the 
organism,  whether  plant  or  animal,  is  comparable  to  the  stoking 
of  the  engine,  but  with  this  difference,  that  the  food  material, 
unlike  the  fuel  in  the  engine  furnace,  has  usually  to  undergo 
complex  chemical  processes,  which  may  actually  result  in  the 
formation  of  new  protoplasm,  before  it  is  available  as  a  source 
of  energy. 

Supplies  of  energy  are,  of  course,  useless  unless  they  can 
be  liberated  when  required.  The  fuel  must  be  burnt,  and 
for  this  purpose,  as  we  have  already  said,  a  supply  of  oxygen 
gas  is  necessary,  and  the  function  which  is  concerned  in  pro- 
viding this  supply  we  call  respiration.  The  term  respiration, 
however,  is  one  in  the  employment  of  which  we  shall  have  to 
exercise  a  certain  amount  of  care.  It  is  naturally  associated 
in  our  minds  with  the  mechanical  act  of  breathing  which  we 
ourselves  perform.  We  can  see  a  man  breathing,  but  we  cannot 
see  an  oak  tree  breathing ;  nevertheless  the  oak  tree  performs 
the  function  of  respiration  just  as  efficiently  as  the  man.  We 
have  to  learn  to  dissociate  the  essential  part  of  this  function, 
which  is  common  to  all  living  things,  from  the  subsidiary  com- 
plications which  have  been  introduced  in  the  case  of  the  higher 
animals  during  the  course  of  their  evolution  from  lower  forms. 

Respiration,  in  the  scientific  acceptance  of  the  term,  is  simply 
the  exchange  by  the  organism  of  the  carbon  dioxide  gas  which 
has  been  formed  in  the  body  in  the  process  of  combustion  for 
the  oxygen  gas  which  is  required  for  that  combustion.  It  is 
therefore  a  double  function — oxygen  being  taken  in  and  carbon 
dioxide  got  rid  of  by  one  and  the  same  process.  This  process 
is,  in  its  essential  features,  an  extremely  simple  one,  depending 
upon  the  physical  principle  of  osmosis  or  diffusion,  in  accordance 
with  which  two  gases  of  different  densities  tend  to  change  places, 
until  equilibrium  is  established,  whenever  they  are  placed  in 
the  necessary  relations  with  one  another,  as  when  they  are 
separated  only  by  some  membrane  through  which  both  can 
pass. 

Carbon  dioxide,  however,  is  not  the  only  waste  product 
resulting  from  the  breaking  up  of  the  complex  proteids,  for 
these  also  contain  hydrogen,  nitrogen,  sulphur  and  phosphorus, 
and  other  products  of  decomposition  are  accordingly  formed 


METABOLISM  9 

which  contain  amongst  them  all  of  these  elements.  These  must 
also  be  eliminated  from  the  body,  and  this  process  of  elimination 
of  waste  products  constitutes  the  function  of  excretion.  This 
function  may  be  performed  in  a  variety  of  ways  and  by  a  variety 
of  organs.  In  so  far  as  the  carbon  dioxide  is  concerned  it  is,  as 
we  have  already  seen,  an  essential  part  of  the  function  of 
respiration.  Urea,  on  the  other  hand,  a  nitrogenous  substance 
which  is  perhaps  the  most  characteristic  waste  product  in  the 
higher  animals,  is  eliminated  by  special  excretory  organs,  such 
as  the  kidneys.  In  the  case  of  the  higher  plants  the  waste 
products  are  for  the  most  part  stored  up  in  the  leaves  and 
got  rid  of  when  these  are  shed. 
We  have  thus  seen  that  the  supply  of  energy  to  a  living 


Living  Protoplasm. 


Food  Material   / \Waste  Products. 

FIG.  1.— Diagram  of  Metabolism. 

organism,  and  therefore  also  its  life,  depends  upon  a  series 
of  complex  chemical  processes  which  take  place  within  the  body. 
All  these  processes  collectively  are  spoken  of  as  metabolism,  and 
we  may  distinguish  between  two  sets  of  metabolic  changes : 
those  which  are  constructive  and  lead  to  the  building  up  of 
new  living  protoplasm  out  of  food  material,  and  those  which 
are  destructive  and  lead  to  the  decomposition  of  the  body 
substance,  the  liberation  of  energy  and  the  formation  of  waste 
products.  The  former  are  termed  anabolic  and  the  latter 
katabolic. 

We  may  roughly  illustrate  these  elementary  conceptions  of 
the  chemical  processes  which  take  place  in  the  living  body  by 
the  accompanying  diagram,  in  which  a  mass  of  living  protoplasm 
is  represented  as  balanced  in  a  very  unstable  position  on  the 
apex  of  a  triangle.  It  is  constantly  undergoing  destruction, 


10         OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

accompanied  by  the  liberation  of  energy  and  resulting  in  the 
formation  of  waste  products,  substances  which,  in  falling  down 
one  side  of  the  triangle  to  a  lower  level,  to  a  greater  or 
less  extent  lose  their  potential  energy.  On  the  other  side  of 
the  triangle,  however,  complex  food  material,  containing  fresh 
supplies  of  potential  energy,  is  supposed  to  be  taken  in,  or,  in 
the  case  of  green  plants,  actually  built  up  from  simple  con- 
stituents by  the  energy  of  the  sun's  rays,  and  used  in  repairing 
the  waste  of  the  protoplasmic  body. 

If  the  constructive  processes  proceed  more  vigorously  and 
rapidly  than  the  destructive,  if  the  food  supply  is  abundant  and 
the  expenditure  of  energy  comparatively  low,  the  body  may 
grow,  though,  as  we  shall  see  presently,  only  within  certain 
limits.  If  the  reverse  is  the  case  and  the  expenditure  exceeds 
the  income,  the  body  may  dwindle  away  and  finally  die.  If  it 
is  to  remain  in  a  condition  of  healthy  equilibrium  a  just  balance 
must  be  maintained  between  the  two  sides  of  the  account. 

Perhaps  the  most  characteristic  property  of  living  things  is, 
as  we  have  already  suggested,  the  power  of  reproduction.  This  is 
the  last  resort  of  the  organism  in  the  struggle  for  existence.  The 
individual,  which  owes  its  very  life  to  the  perishable  nature  of 
its  body,  always  succumbs  to  the  destructive  influences  of  the 
environment  sooner  or  later,  but  before  yielding  to  the  inevitable 
it  will,  under  normal  conditions,  have  produced  offspring  which 
will  carry  on  the  struggle  for  another  generation. 

The  phenomenon  of  reproduction  is  intimately  associated 
with  that  of  growth,  and  may  be  traced  back  to  the  division  of 
a  simple  ancestral  mass  of  protoplasm  into  two  parts  whenever 
its  size  increased  to  such  an  extent  that  the  ratio  of  surface  to 
volume  became  too  small  for  the  necessary  intercourse  between 
the  organism  and  its  environment.  With  this  division  of  the 
protoplasmic  body  the  proper  proportion  is  restored,  and  hence 
reproduction  by  multiplication  of  protoplasmic  units  may  be 
looked  upon  as  primarily  the  direct  consequence  of  super- 
abundant nutrition. 

We  have  now  learnt  to  look  upon  an  animal  or  a  plant  as  a 
complex  and  extremely  delicate  piece  of  mechanism;  constantly 
employed  in  collecting  energy  directly  or  indirectly  from  the 
sun's  rays  and  in  using  that  energy  to  maintain  an  incessant 
struggle  against  the  destructive  forces  of  its  environment.  This 
incessant  getting  and  spending,  winning  and  losing,  constitutes 


THE   NATURE   OF   LIFE  11 

what  we  call  the  life  of  the  organism.  In  considering  what  is 
the  meaning  of  all  this  we  must  remember  that,  primarily  at  any 
rate,  every  living  thing  exists  for  its  own  benefit,  and  that 
living,  like  virtue,  is  its  own  reward.  The  organism  also, 
however,  exists  for  the  benefit  of  future  generations,  to  which 
it  may  hand  on  the  lamp  of  life  before  its  own  flame  is  finally 
extinguished.  There  is  a  race  life  as  well  as  an  individual  life, 
and  we  cannot  realize  too  clearly  that  in  the  economy  of  nature 
the  former  is  of  infinitely  greater  importance  than  the  latter. 

The  ideas  which  we  have  just  been  considering  are  by  no 
means  of  modern  origin.  More  than  three  centuries  ago  the 
philosopher  Descartes  endeavoured  to  explain  the  human  body 
as  a  machine,  but  as  a  machine  under  the  control  of  the  "  soul," 
which  he  curiously  located  in  that  part  of  the  brain  known  as 
the  pineal  gland.  His  ideas  of  physiology,  however,  were, 
naturally,  of  the  crudest  description,  and  immense  strides  have 
been  made  in  this  direction  since  his  time.  Chemists  and 
physicists  have  helped  us  much  towards  a  correct  understanding 
of  the  living  mechanism,  but  when  they  have  done  their  best  it 
may  well  be  that  the  question  "  What  is  Life  ?  "  will  still  remain 
unanswered,  and  that  we  may  still  have  to  take  refuge  in  the 
idea  of  an  unknown  "  soul "  to  explain  the  difference  between 
living  and  not-living  things.  The  "soul"  of  Descartes' 
philosophy  corresponds  more  or  less  closely  with  the  "  vital 
force  "  of  some  more  recent  writers  and  the  "  entelechy  " l  of 
others,  but  whatever  term  we  employ  it  must  be  rather  as  a 
cloak  for  our  ignorance  than  as  an  expression  of  any  definite 
opinion  as  to  what  it  is  that  really  animates  the  living  body. 

1  Vide    Driesch,    "  The   Science  and   Philosophy  of  the    Organism,"   (London 
A.  &  C.  Black,  1908),  Vol.  2,  pp.  137,  138. 


CHAPTEK   II 

Amoeba  as  a  typical  organism — The  properties  of  protoplasm. 

IN  illustration  of  the  general  principles  dealt  with  irffhe  fore- 
going chapter  we  may  now  consider  a  definite  j^licrete  example 
of  a  living  organism.  Probably  none  is  better  suited  for  this 
purpose  than  the  familiar  Amoeba,  which  may  be  regarded  as  a 
kind  of  pocket  edition  of  a  typical  animal. 

Amoebae  may  be  found  creeping  about  on  the  mud  at  the 
bottom  of  ponds  and  ditches.  Although  of  microscopic  size  and, 
usually  at  any  rate,  invisible  to  the  raked  eye,  they  are  by  no 
means  the  smallest  or  simplest  of  living  things,  but  exhibit 
within  the  narrow  limits  of  their  gelatinous  bodies  a  considerable 
amount  of  structural  differentiation. 

In  general  appearance  (Fig.  2)  an  Amoeba  resembles  nothing; 
so  much  as  an  irregular  speck  of  translucent  jelly,  but  if  we, 
watch  it  for  a  few  minutes  under  the  microscope,  we  soon  find, 
that  it  is  something  more  than  this.  If  in  an  active  and 
healthy  condition  it  never  maintains  the  same  shape  for  long 
together,  but  manifests  an  ever-changing  irregularity  as  it 
slowly  creeps  about  from  place  to  place,  throwing  out 
irregular  projections  of  its  body  first  in  one  direction  and 
then  in  another  and  withdrawing  old  projections  as  new  ones 
are  put  forth. 

The  viscid  substance  of  which  the  entire  body  is  composed  is 
protoplasm,  but  this  protoplasm  is  not  homogeneous  throughout ; 
on  the  contrary,  it  exhibits  a  characteristic  differentiation  into 
parts  or  organs,  which  can  be  more  or  less  readily  distinguished 
from  one  another  and  each  of  which  has  its  own  duties  or 
functions  to  perform. 

Inasmuch  as  it  consists  of  but  one  protoplasmic  unit,  however, 
we  may  speak  of  the  body  of  an  Amoeba  as  a  single  cell.1  As 
in  all  other  typical  cells,  the  most  fundamental  differentiation 

1  The  origin  and  meaning  of  this  term  will  be  discussed  more  fully  in  a  later 
chapter. 


AMCEBA 


13 


which   it  shows  is  into  cell-body  and  nucleus.     The   cell-body 
forms    by    far    the    greater    part   of    the   organism,  and    the 


D. 


PIG.  2. — Amoeba. 

A,  S.    I  he  game  individual  in  two  phases  of  active  movement,  showing  change  of  form. 

C.  Another  specimen,  of  a  different  species.     Note  the  numerous  short  projections  at 

the  hinder  end  due  to  contraction,  while  at  ect.  the  commencement  of  a  new  pseudo- 
podium  is  indicated  by  a  thickening  of  the  ectoplasm. 

D.  A  specimen  with  two  nuclei. 

E.  Diagram  of  reproduction  by  simple  fission. 

c.v.  Contractile  vacuole ;  ect.  ectoplasm  or  ectosarc;  end.  endoplasm  or  endosarc ;  f.p. 
food  particles;  f.v.  food  vacuole;  nu.  nucleus ;  psd.  pseudopodium.  (The  arrows 
show  the  general  direction  in  which  the  animal  is  moving.) 


protoplasm  of  which   it  is  composed  is  often  distinguished  as 
cytoplasm. 


14          OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

The  nucleus  (Fig.  2,  nu.),  composed  of  a  somewhat  different 
variety  of  protoplasm  sometimes  known  as  karyoplasm  or 
nucleoplasm,  is  a  very  definite  body  of  more  or  less  rounded 
form,  sometimes  shaped  like  a  bun,  and  easily  distinguishable 
from  the  cytoplasm  even  in  the  living  animal  by  its  more  highly 
refractive  character.  Its  position  is  by  no  means  constant,  for 
it  floats  about  from  place  to  place  in  the  interior  ofjthe  almost 
liquid  cell-body. 

The  cytoplasm  is  very  imperfectly  differentiated  into  infeer  and 
outer  portions.  The  former,  in  which  the  nucleus  is  lodged,  is 
often  called  the  endosarc  or  endoplasm  (Fig.  2,  end.),  the  latter 
the  ectosarc  or  ectoplasm  (Fig.  2,  ect.).  The  ectoplasm  must  be 
regarded  as  a  feebly  developed  protective  layer  ;  it  is  the  part  which 
comes  into  direct  relation  with  the  surrounding  water  and  through 
which  all  intercourse  between  the  Amosba  and  its  environment 
must  take  place.  Though  soft  and  gelatinous,  it  is  a  good  deal 
firmer  and  denser  than  the  endoplasm,  and  &4s-atstr-more  trans- 
parent, for  the  endoplasm  contains  imbedded  in  it  numerous  more 
or  less  opaque  particles  of  various  kinds  which  give  it  a  coarsely 
granular  character.  Most  of  these  particles  are  minute,  but 
others  are  generally  present  of  comparativel^Jarga  size~  and 
enclosed  in  drops  of  clear  liquid.  These  latter  are  food  particles 
(Fig.  2,  f.p.)  undergoing  digestion,  and  they  can  frequently  be 
identified  as  the  bodies  of  other  organisms  which  the  Amosba 
has  taken  in,  microscopic  plants  or  animals  smaller  than  itself. 
The  drops  of  liquid  in  which  they  occur  are  termed  foo$ 
vacuoles  (Fig.  2, /.#.). 

Another  spherical  drop  of  liquid  (Fig.  2,  c.v.)  may  be  observed 
somewhere  near  the  surface  of  the  cell-body.  This  is  perfectly 
clear  and  contains  no  solid  particles ;  moreover  it  undergoes  a 
rhythmical  dilatation  and  contraction,  gradually  increasing  to  a 
maximum  size  and  then  suddenly  disappearing  owing  to  the 
discharge  of  its  contents  into  the  surrounding  water.  If  the 
spot  where  this  "  contractile  vacuole  "  disappears  be  carefully 
watched  another  drop  of  liquid  is  seen  gradually  to  accumulate 
there,  and  the  process  is  repeated. 

We  are  told  that  in  the  early  days  of  chemistry,  before  the 
highly  specialized  apparatus  which  is  now  used  was  thought  of, 
the  originator  of  the  atomic  theory  performed  his  experiments 
with  the  ordinary  domestic  crockery.  So  also  an  Amosba  is  able 
to  perform,  with  the  extremely  simple  apparatus  at  its  disposal, 


LOCOMOTION   IN   AMCEBA  15' 

all  those  actions  or  functions  which  are  really  essential  for  the 
maintenance  of  life. 

In  the  higher  animals  the  primary  differentiation  of  the  body 
is  into  an  outer  protective  and  an  inner  digestive  layer,  each  of 
very  complex  structure.  The  Amoeba  accomplishes  the  same 
end  in  its  own  primitive  manner  by  the  differentiation  into 
ectoplasm  and  endoplasm.  In  most  of  the  higher  animals, 
again,  we  find  very  well  developed  organs  of  locomotion  in 
the  form  of  limbs.  The  Amoeba  has  no  permanent  organs  of 
locomotion  at  all  but  merely  temporary  projections  of  the  body, 
the  so-called  pseudopodia  (Fig.  2,  psd.),  which  arr  put  forth  when 
required;  In  both  cases,  however,  movement  is  effected  in 
essentially  the  same  way,  by  "contraction  and  expansion  of  the 
living  protoplasm.  In  the  higher  animals  this  power  of  con- 
traction is  localized  in  the  muscles,  which  are  highly  specialized 
for  the  purpose  and  have  no  other  duties  to  perform,  while  in 
the  Amoeba  any  -part  of  the  body,  or  at  any  rate  of  the  ectoplasm, 
may  contract  or  expand  as  occasion  requires. 

In  the  process  of  formation  of  a  new  pseudopodium  we  see* 
first  a  thickening  and  protrusion  of  the  clear  ectoplasm  (Fig.  "2, 
C.,  ect.),  accompanied  by  a  streaming  in  of  the  endoplasm,  and 
the  latter  seems  to  bulge  out  the  ectoplasm  as  it  flows  forwards. 
The  pseudopodium  is  withdrawn  again  by  a  reversal  of  the  pro- 
cess, the  endoplasm  streaming  out  from  it  into  the  central  ~>ass. 
of  cytoplasm  and  the  ectoplasm  contracting  after  the^retreating 
endoplasm.     Thus  at  the  posterior  end  of  an  activefjf  creeping  * 
Amoeba  one  frequently  sees  numerous  blunt  proiections  Avhich  are 
the  last  remnants  of  retracted  pseudopodia  (Fig.  *,  o.;.     ^. 
shape  of  the  pseudopodia  when  fully  extended  differs  very  //>• 
in  different  kinds  of  Amoebae.     In  some  species  they  are'7 
paratively  short,  thick  and  blunt,  as  in  our  illustration,  whJ( 
others  they  are  very  long  and  slender,  radiating  outwards 
the  body  of  the  animal  in  all  directions.     They  are  used  bj 
possessor  not  only  as  organs  of  locomotion  but  also  as  I 
organs  and,  as  we  shall  see  directly,  for  the  capture  of  prey 

The  protrusion  and  retraction  of  pseudopodia  imply,  of  an 
the  expenditure  of  energy,  and  this  energy  must  be  derived  : 
the  combustion  of  the  body  of  the  Amoeba.  In  this  waj 
protoplasm  is  gradually  used  up,  and,  unless  the  animal  is  Ci 
of  starvation,  it  must  be  replaced,  which  brings  us 
consideration  of  the  manner  in  which  an  Amoeba  perfor 


16          OUTLINES   OF   EVOLUTIONARY  BIOLOG 

important  function  of  nutrition.  As  it  crawls  slowly  about  the 
pseudopodia  come  into  contact  with  all  sorts  of  solid  particles  in 
the  surrounding  water.  Some  of  these  will  be  inorganic,  grains  of 
sand  and  so  forth,  others  will  be  the  dead  or  living  bodies  of  other 
organisms,  sometimes  much  smaller  than  the  Amoeba  itself. 
The  Amoeba  has  the  remarkable  power  of  distinguishing  amongst 
these  different  kinds  of  particles  those  which  are  good  for  food 
from  those  which  are  not.  How  it  does  so  we  do  not  know  ;  we 
can  only  say  that  the  presence  of  a  particle  which  is  good  for 
food  stimulates  the  living  protoplasm  in  a  way  quite  different 
from  that  in  which  it  is  stimulated  by  the  presence  of  a  mere 
grain  of  sand.  In  the  latter  case  the  Amoeba  will  simply  pass  to 
one  side  and  avoid  the  object ;  in  the  former  it  will  put  forth 
pseudopodia  which  will  .close  around  and  envelop  it.  The  food 
particle  is  thus  passed  through  the  ectoplasm  into  the  interior 
of  the  body.  There  is  no  definite  mouth,  but  food  is  taken  in 
wherever  it  happens  to  come  into  contact  with  the  surface  of  the 
body,  and  the  aperture  closes  up  after  it.  Thus  a  temporary 
mouth,  is  formed  as  occasion  demands.  Similarly  there  is  no 
permanent  digestive  cavity  or  stomach,  but  merely  a  temporary 
food  vacuole  into  which  a  digestive  fluid  is  doubtless  secreted 
by  the  surrounding  protoplasm.  ^  • 

Digestion,  as  in  higher  animals,  is  essentially  a  process  of 
soliif'oii,  whereby  those  parts  of  the  food  which  are  digestible  are 
dissolved  and  *r-endered  capable  of  diffusing' from  the  digestive 
cavity  into  the  surrounding  body.  The  higher  animals  make 
use  chiefly  of  three  classes  of  food  material,  proteids,  carbo- 
hydrates (e.g.,  starches -and  sugars)  and  fats.  It  is  said  that 
Amoeba  can  only  digest  proteids,  which  of  course  it  must  obtain 
from  the  protoplasmic  bodies  of  other  organisms.-  When  diges- 
tion is  complete  a  certain  amount  of  insoluble  residues  from  the 
food  will  remain  over ;  these  constitute  the  faeces  and  have  to  be 
got  rid  of.  There  is,  however,  no  permanent  vent  or  anus,  and 
the  faeces  are  cast  out  through  the  ectoplasm  at  the  hinder  end 
of  the  body  as  the  animal  crawls  along. 

Owing  to  the  minute  size  of  the  whole  organism  there  is  no 
need  for  a  complex  circulatory  system,  such  as  is  found  in  the 
higher  animals,  for  the  distribution  of  the  digested  food;  it  merely 
soaks  into  the  surrounding  protoplasmic  body  from  the  food 
vacuoles,  and,  by  anabolic  changes  which  are  not  fully  under- 
stood, is  converted  into  new,  living  protoplasm. 


INSPIRATION   IN  AMCEBA  17 


It  is  not  easy,  if  it  be  possible  at  all,  actually  to  observe  the 
process  of  respiration  in  so  small  an  animal  as  an  Amoeba,  but 
we  know  perfectly  well  from  the  analogy  of  higher  organisms 
what  must  take  place.  Oxygen  is  required  for  the  combustion  of 
the  protoplasm  from  which  the  energy  of  the  organism  is 
derived,  and  this  oxygen  occurs  in  a  state  of  solution  in  all 
ordinary  water  which  is  exposed  to  the  air.  At  the  same  time 
carbon  dioxide,  or  carbonic  acid  gas,  must  be  produced  as  one  of 
the  products  of  the  combustion,  by  oxidation  of  the  carbon  in  the 
protoplasm.  This  waste  product  (C02)  will  first  of  all  be 
dissolved  in  the  water  which  forms  the  greater  part  of  the  bulk 
of  the  living  organism,  while  at  the  same  time  the  water  by 
which  the  animal  is  surrounded  may  be  regarded  as  a  very 
dilute  solution  of  oxygen.  The  outermost  layer  of  the  ectoplasm 
may  be  looked  upon  as  a  thin  membrane  separating  the  two 
solutions. 

We  know  from  experiment  that  whenever  two  gases,  or  solutions 
of  gases,  of  different  density,  are  separated  from  each  other  by  a 
thin  organic  membrane,  they  will  pass  through  that  membrane 
in  opposite  directions  until  a  state  of  equilibrium  is  established 
between  the  two.  This  process  of  osmosis  or  diffusion  is,  as  we 
have  already  seen,  the  essential  feature  of  respiration  in  pi  plants 
arid  animals,  although  probably  the  purely  physical  process  is 
Controlled  to  some  extent  by  the  living  protoplasm. 

In  the  case  of  the  Amoeba  then,  the  carbon  Dioxide  diffuses 
out  through  the  ectoplasm  into  the  surrounolng  water  while 
the  oxygen  from  the  surrounding  water  diffuses  in,  and  the 
necessary  exchange'  of  gases  is  brought  about.  No  specialized 
organs  of  respiration,  such  as  we  meet  with  in  the  higher 
animals,  are  required.  The  whole  surface  is  a  respiratory  sur- 
face, and  all  parts  of  the  interior  are  within  reach  by  the  simple 
process  of  diffusion,  aided  doubtless  by  the  circulation  of  the  semi- 
liquid  protoplasm  which  is  constantly  going  on  inside  the  body. 
-"'Other  waste  products  must  be  formed  by  the  combustion  of  the 
protoplasm_in  addition  to  carbon  dioxide.  What  these  are  we 
do  not  exactly  know  in  the  case  of  the  Amoeba,  but  it  is  evident 
that  they  must  contain  nitrogen,  which  is  one  of  the  essential 
constituents  of  all  proteids.  These  waste  products  must  be  got 
rid  of  by  some  process  of  excretion,  and  it  is  usually  supposed 
that  they  are  passed  in  a  state  of  solution  to  the  contractile 
vacuole  and  thence  periodically  expelled  to  the  exterior,  so  that 

B.  c 


18    OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

the  contractile  vacuole  is  probably  to  be  regarded  as  the  special 
excretory  organ  of  the  animal. 

In  all  but  the  lowest  animals  there  is  a  more  or  less  specialized 
jiervous  system,  whose  function  it  is  to  place  the  different  parts 
^of  the  body  in  communication  with  one  anothex_andr  through 
the  mediation  of  the  sense  organs,  with  the  external  world  or 
environment.^  The  action  of  this  system  depends  primarily 
upon  one  of  the  fundamental  properties  of  living  protoplasm, 
the  power  of  responding  to  stimuli  by  some  definite  change 
in  its  own  condition.  The  stimulus  is,  in  the  first  instance, 
supplied  by  some  factor  of  the  environment,  such  as  light,  heat, 
electricity  or  mechanical  impact.  It  may  appear  to  originate  in 
the  central  nervous  system  itself,  but  this  is  probably  secondary. 
It  has  the  effect  of  liberating  stored  energy  in  those  parts  of 
the  organism  which  are  sensitive  to  that  particular  stimulus, 
in  somewhat  the  same  way  that  the  stimulus  of  heat  may 
have  the  effect  of  liberating  the  stored  energy  in  a  charge  of 
gunpowder.  The  living  molecule  has,  indeed,  actually  been 
described  as  explosive.  In  both  cases  potential  energy  is 
converted  into  kinetic  energy,  and  the  effect  which  is  produced 
may  be  out  of  all  proportion  to  the  amount  of  energy  represented 
by  the  liberating  stimulus  itself. 

In  the  higher  animals,  then,  the  stimulus  received  from  the 
external  environment,  whatever  its  nature  may  be,  acts 
primarily  upon  some  special  sense  organ  or  receptor,  whence  it 
is  transmitted  along  highly  specialized  tracts  of  tissue,  the 
nerves,  to  some  part  of  the  central  nervous  system,  where  it 
usually  gives  rise  to  what  we  call  a  sensation.  The  central 
nervous  system,  again,  not  only  has  the  power  of  receiving  stimuli 
tli rough  afferent  or  sensory  nerves,  but  also  of  sending  stimuli 
through  ^efferent  nerves  to  the  various  organs  of  the  body, 
whereby  their  functions  are  controlled  and  regulated.  The  con- 
traction of  muscles  and  the  secretion  of  glands  are  all  con- 
trolled in  this  manner,  and  the  entire  working  of  the  body  is 
co-ordinated  by  the  action  of  the  nervous  system. 

In  the  Amoeba,  however,  we  see  no  trace  of  a  special  nervous 
system,  nor  of  sense  organs,  but  nevertheless  the  organism  is 
certainly  capable  of  receiving  and  responding  to  stimuli ;  in 
other  words  it  is  irritable.  Thus  the  protoplasmic  body  responds 
by  contraction  to  the  stimuli  of  mechanical  impact,  heat,  light, 
electricity  and  chemical  reagents,  and,  as  we  have  already  seen, 


VITALISM  19 

the  presence  of  particles  which  are  good  for  food  causes  the  pro- 
trusion of  pseudopodia  in  a  definite  and  purposive  manner. 
Probably  the  whole  of  at  least  the  ectoplasm  is  to  some  extent 
sensitive  to  stimuli  of  certain  kinds,  and  it  is  also  probable  that 
stimuli  may  be  conducted  from  one  part  of  the  body  to  another 
without  the  existence  of  special  nervous  tracts. 

One  of  the  most  difficult  problems  in  connection  with  the 
physiology  of  Amoeba — and  indeed  of  any  living  organism — is 
that  of  automatism.  Does  an  Amreba  do  anything  really  auto- 
matically or  spontaneously,  or  are  all  its  actions  the  result, 
direct  or  indirect,  of  the  application  of  external  stimuli  to  the 
explosive  molecules  of  living  matter  ?  Is  the  organism  merely  a 
machine  run  by  the  environment,  or  is  it  something  more  ?  Here, 
of  course,  at  the  very  beginning  of  our  investigations,  we  are  face 
to  face  wifeh  the  old  question,  already  referred  to,  of  the  existence 
of  an  animating  principle  or  "  soul,"  which  exercises  some  sort  of 
control  over  the  physical  and  chemical  processes  upon  which  the  life 
of  the  organism  depends./-  This  is  a  question  which  perhaps  falls 
within  the  province  of  the  philosopher  rather  than  that  of  the 
biologist.  The  theory  of  vitalism,  by  postulating  the  existence 
of  some  such  special  vital  force  in  all  living  things,  undoubtedly 
enables  us  to  avoid  many  difficulties,  but  it  is  doubtful  if  it  really 
explains  anything.  As  a  matter  of  fact  the  more  we  study  living 
organisms  by  actual  observation  and  experiment,  the  more 
fully  are  we  able  to  interpret  their  behaviour  in  terms  of 
chemistry  andph^sics^but  this  is  a  very  different  thing  from 
saying  that^cnemistry  and  physics  will  ultimately  yield  a 
complete  explanation  of  vital  phenomena. 

It  is  quite  possible,  for  example,  that  the  movements  of  the 
Amceba  may  all  ultimately  be  interpreted  in  such  terms,  for 
Biitschli  has  shown  that  they  can  be  closely  imitated  by  minute 
artificially  prepared  drops  of  oil-foam  surrounded  by  water.  The 
substance  of  which  these  droplets  are  composed  is  of  course 
totally  different  chemically  from  protoplasm  and  is  in  no  sense 
alive,  but  it  seems  highly  probable,  if  not  certain,  that  since 
purely  physical  processes  (amongst  which  surface  tension  seems 
to  play  an  important  part)  are  capable  of  producing  strikingly 
amoeboid  movements  in  the  oil-foam,  they  may  also  be  largely, 
if  not  solely,  responsible  for  the  similar  phenomena  of  movement 
in  the  living  protoplasm  of  the  Amceba  itself,  which  seems  closely 
to  resemble  an  oil-foam  in  its  physical  properties. 

c  2 


20          OUTLINES   OF   EVOLUTIONAEY  BIOLOGY 

Like  other  organisms,  the  Amoeba  sometimes  undergoes  a  period 
of  rest)  during  which  its  various  activities  are  more  or  less  com- 
pletely suspended.  Under  these  circumstances  the  pseudopodia 
are  withdrawn,  the  body  is  rounded  off  and  a  protective  envelope 
or  cyst  is  secreted  by  the  protoplasm.  This,  however,  is  only  a 
temporary  state,  perhaps  necessitated  by  unfavourable  conditions 
of  the  environment,  and  sooner  or  later  the  organism  emerges 
from  its  retirement  and  resumes  its  activity. 

If  in  the  course  of  its  wanderings  the  Amoeba  meets  with  an 
abundant  supply  of  food  and  takes  in  more  than  is  actually 
required  to  make  good  the  waste  of  protoplasm ;  if,  in  other 
words,  anabolism  preponderates  over  katabolism,  the  organism 
may  increase  in  size  by  growth,  by  the  addition  of  new  particles 
of  protoplasm  in  excess  of  those  which  are  used  up.  These  new 
particles  are  deposited,  not  on  the  surface,  but  throughout  the 
whole  mass  of  protoplasm,  between  those  which  are  already 
formed.  Thus  growth  takes  place,  not  by  accretion,  as  in  a 
crystal  or  a  snowball,  but  by  intussusception,  and  we  have  here 
a  characteristic  though  by  no  means  absolute  distinction  between 
the  growth  of  not-living  and  that  of  living  matter. 

As  in  all  organisms,  however,  there  is  a  limit  to  the  size  which 
the  body  may  attain,  and  this  limit  varies  with  different  species 
of  Amoeba.  It  depends  primarily,  no  doubt,  upon  the  necessary 
relation  between  surface  and  volume.  As  we  have  seen,  all 
interchange  between  the  organism  and  its  environment  has  to  be 
maintained  through  the  surface,  and  a  given  area  of  surface 
cannot  supply  the  wants  of  more  than  a  certain  volume  of 
protoplasm.  As  the  animal  grows  the  volume  must  necessarily 
increase  in  a  much  higher  ratio  than  the  surface,  and  the  pro- 
portion between  the  two  is  rapidly  altered.  This  is  probably 
not  the  whole  explanation  of  the  limitation  of  growth  in  an 
Amoeba,  the  problem  being  doubtless  complicated  by  other  factors, 
but  we  may  take  it  as  quite  certain  that  increase  beyond  a  certain 
size,  if  possible,  would  inevitably  result  in  death.  Such  a 
calamity  is  avoided  by  the  simple  expedient  of  dividing  into  two 
parts  whenever  the  limit  of  safety  is  reached.  The  nucleus 
divides  first  and  the  two  halves  move  away  from  one  another, 
then  the  protoplasm  constricts  into  a  bridge  between  the  two 
nuclei,  the  bridge  narrows  and  finally  ruptures,  and  instead  of 
one  Amoeba  there  are  now  two,  each  exactly  resembling  the 
parent  (Fig.  2,  E.). 


PHYSICAL   PROPERTIES  OP  PROTOPLASM         21 

This  simple  fission  of  a  single  protoplasmic  unit,  or  cell,  forms, 
as  we  shall  see  later  on,  the  essential  feature  of  ordinary  repro- 
duction throughout  the  animal  and  vegetable  kingdoms.  It  will 
be  observed  that  in  this  process  generation  succeeds  generation 
without  the  intervention  of  anything  which  we  can  speak  of  as 
death.  There  is  indeed  no  room  for  death  in  the  history  of 
these  simple  organisms,  unless  it  be  death  by  accident,  for  every 
time  fission  takes  place  the  entire  body  is  used  up,  and  nothing  is 
left  over  to  die.  Nor  is  there  any  distinction  to  be  drawn  between 
parent  and  offspring,  for  the  two  new  individuals  are  in  all 
respects  similar  to  one  another,  and  neither  can  be  said  to  precede 
the  other  in  point  of  time. 


We  have  now  become  sufficiently  well  acquainted  with  the 
nature  of  protoplasm  to  profit  by  a  more  detailed  examination  of 
its  physical  and  chemical  properties.  We  have  seen  that,  as  it 
occurs  in  the  body  of  an  Amoeba,  it  is  a  viscid,  more  or  less 
liquid,  colourless  substance,  almost  transparent  but  exhibiting, 
under  moderately  high  powers  of  the  microscope,  a  granular 
appearance  due  to  the  presence  of  numerous  minute  and  more  or 
less  opaque  particles.  These  particles  may  be  regarded  as 
impurities,  and  indeed  protoplasm  can  never  be  obtained  in  a 
perfectly  pure  state,  for  it  is  constantly  undergoing  chemical 
change,  both  constructive  and  destructive,  and  the  impurities 
owe  their  origin  partly  to  the  food  materials  which  are  taken  in 
and  partly  to  katabolic  processes  which  give  rise  ultimately  to 
waste  products. 

Even  apart  from  these  impurities,  however,  the  protoplasm 
itself  never  exhibits  a  perfectly  uniform  structure.  It  is  by  no 
means  homogeneous  but  shows  a  more  or  less  distinct  differentia- 
tion into  different  portions,  as,  for  example,  into  nucleoplasm  and 
cytoplasm,  ectoplasm  and  endoplasm,  and  so  forth.  In  other 
words  it  is  an  organized  substance.  Moreover,  it  has  a  character- 
istic minute  structure  or  texture  which  can  to  some  extent  be 
recognized  under  high  powers  of  the  microscope  and  concerning 
the  interpretation  of  which  different  observers  are  as  yet  by  no 
means  all  agreed.  According  to  Professor  Biitschli  it  is  a  kind 
of  microscopic  foam,  consisting  of  exceedingly  minute  drops  of 
a  more  liquid  substance  separated  by  very  thin  layers  of  denser 
material,  and  thus  resembling  physically  such  a  foam  as  can 
be  prepared  from  a  mixture  of  oil,  salts  of  various  kinds,  and 


22    OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

water.  If  this  really  be  a  correct  account  of  the  minute  structure 
of  living  protoplasm  it  helps  us,  as  we  have  already  seen,  to 
explain  its  characteristic  movements  in  terms  of  well  known 
physical  phenomena.  </ 

Other  competent  observers,  however,  maintain  that  the  appear- 
ance of  foam-structure  is  a  delusion  and  that  what  Biitschli 
interprets  as  thin  sheets  separating  the  droplets  from  one  another 
are  in  reality  very  delicate  fibres  arranged  in  a  network.  These 
fibrillae  are  supposed  to  be  contractile  and  thus  to  be  responsible 
for  the  movements  of  the  protoplasm  as  a  whole.  But  whence 
comes  the  contraction  of  the  fibrillse  ? 

Various  considerations,  again,  and  especially  the  phenomena 
of  heredity,  oblige  us  to  postulate  for  protoplasm  an  even  more 
minute  fundamental  structure  than  the  microscope  is  capable  of 
revealing  to  us.  It  is,  in  all  probability,  made  up  of  ultra- 
microscopic  material  units,  each  composed  of  a  group  of  molecules, 
which  units,  or  "biophors,"  must  themselves  be  regarded  as  living 
bodies  capable  of  nourishing  themselves,  growing  and  multiplying 
by  division. 

It  is  difficult  to  form  a  satisfactory  idea  of  the  chemical  com- 
position of  protoplasm  because  it  is  impossible  to  analyze  it  in  the 
living  condition  ;  indeed,  in  the  living  condition  it  is  constantly 
undergoing  chemical  change,  and  the  moment  it  dies  it  ceases  to 
be  protoplasm.  •  It  is  certain,  however,  that  it  is  not  a  definite 
chemical  compound,  but  a  mixture  of  several  distinct  sub- 
stances :  proteids,  mineral  salts  and  water.  Moreover,  different 
samples  of  protoplasm,  taken  from  different  organisms  or  from 
different  parts  of  the  same  organism,  may  differ  widely  from  one 
another  in  chemical  composition.  Thus  the  difference  between 
nucleoplasm  and  cytoplasm  is  largely  a  chemical  one,  depending 
to  some  extent  upon  the  relatively  large  amount  of  phosphorus 
present  in  the  former. 

By  far  the  most  characteristic  and  important  of  the  chemical 
constituents  of  protoplasm  are  the  proteids  (proteins).  These 
form  a  remarkable  class  of  substances  which  do  not  occur  in 
nature  except  in  the  bodies  of  plants  and  animals.  They  are 
definite  chemical  compounds  containing  the  elements  carbon, 
hydrogen,  oxygen,  nitrogen,  sulphur  and  frequently  phosphorus, 
and  they  have  an  extremely  complex  and  unstable  constitution, 
readily  splitting  up  on  oxidation  into  simpler  and  more  stable 
compounds  and  thereby  liberating  kinetic  energy.  Many 


CHEMICAJj   COMPOSITION   OF   PROTOPLASM      23 

different  kinds  of  proteids  are  known  to  us  and  have  received 
special  names ;  such  are  the  albumen  which  occurs  in  the  white 
of  an  egg,  the  casein  which  is  met  with  in  cheese,  the  legumin 
which  is  characteristic  of  peas  and  beans,  the  gliadin  and 
glutinin  of  flour,  and  so  forth. 

These  proteids  are,  for  the  most  part  at  any  rate,  colloid 
substances,  that  is  to  say  they  are  more  or  less  gelatmo^is  and 
incapable  of  diffusing  through  organic  membranes.^This  may 
be  accounted  for  if  we  assume  that  the  highly  complex  molecules 
of  which  they  are  composed  are  too  large  to  pass  through  the 
very  minute  pores  which  occur  in  such  membranes  and  which 
readily  allow  of  the  passage  of  the  molecules  of  simpler, 
crystalloid  substances.  The  colloid  nature  of  the  proteid  con- 
stituents of  protoplasm  plays  a  very  important  part  in  determining 
its  properties  and  -behaviour.  Crystalloid  mineral  salts  and  other 
diffusible  substances  in  a  state  of  solution  can  pass  through  a 
cell-wall  or  membrane  by  osmosis,  and  thus  the  living  proto- 
plasm receives  fresh  supplies  of  nutriment,  but  the  colloid 
proteids  are  as  a  rule  formed  inside  the  cell  and  cannot  usually 
pass  out  again  until  they  have  undergone  some  chemical  change 
whereby  they  are  rendered  diffusible. 

The  mineral  salts  which  we  find  in  the  protoplasm,  usually  in 
a  state  of  solution,  are  of  very  various  kinds,  compounds  of 
sodium,  potassium,  calcium  and  other  elements  with  various 
inorganic  and  organic  acids,  such  as  sulphuric,  hydrochloric, 
malic  arid  citric  acids. 

Finally,  water  must  always  be  present  in  living  protoplasm  and 
usually  forms  a  very  large  percentage  of  the  whole  mass. 

Whatever  view  we  may  take  with  regard  to  the  question  of 
vitalism,  there  can  be  no  doubt  that  the  most  distinctive  property 
of  living  protoplasm  is  its  power  of  controlling  chemical  and 
physical  processes  so  as  to  make  them  yield  results  different  from 
those  which  would  be  obtained  if  we  were  dealing  with  not-living 
matter.  The  various  processes  upon  whicE  defend  the  functions 
of  movement,  nutrition,  respiration  and  excretiorhetll  appear  to  be 
controlled  in  this  manner,  but  the  general  principle  is  perhaps 
most  beautifully  illustrated  in  the  case  of  many  of  the  lower 
animals  and  plants,  in  which  the  protoplasm  secretes  a  protective 
or  supporting  skeleton  of  some  mineral  substance,  such  as  silica, 
or  carbonate  of  lime.  Silica,  for  example,  in  the  inorganic  world, 
occurs  abundantly  in  a  state  of  solution  in  water,  from  which 


OU' 


24          OUTLINES   OF   EVOLUTIONABY   BIOLOGY 

may  be  deposited  in  different  forms,  in  shapeless  masses  as  in 
the  case  of  flints,  or  in  symmetrical  crystals  as  in  some  specimens 


FIG.  3. — Different  forms  of  Radiolarian  Skeletons.  In  the  central  figure  the 
protoplasmic  pseud opodia  are  seen  coming  out  from  the  openings  in  the 
shell.  (From  Haeckel's  "  Kunstformen  der  Natur.") 

of  quartz,   whilst   the   beautiful   opal,   chemically  speaking,   is 
merely  a  hydrate  of  silica,  or  silicic  acid. 


SELECTIVE   POWEKS   OF   PROTOPLASM 


25 


Many  simple  unicellular  organisms,  such   as   the   Radiolaria 
(Fig.  3)  amongst  animals,  and  the  diatoms  amongst  plants,  have 


FIG.  .4. — Different  forms  of  Foraminiferan  Skeletons.     (Frnm  Haeckel's 
"  Kunstformen  der  Natur.") 

the  power  of  taking  up  dissolved  silica  from  the  water  in  which 
they  live  and  using  it  for  building  skeletons.     These  skeletons, 


2G         OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

however,  which  are  really  composed  of  opal,  do  not  consist  either 
of  shapeless  masses  or  of  geometrical  crystals,  but  assume  beauti- 
fully symmetrical  forms  which  vary  with  each  particular  kind  of 
organism  and  which  are  wholly  different  from  any  forms  occurring 
in  the  inorganic  world.  The  same  is  true  of  those  somewhat 
more  highly  organized  members  of  the  animal  kingdom,  the 
siliceous  sponges,  whose  skeletons  consist  of  spicules  of  opal 
(Fig.  88),  often  of  most  beautiful  and  characteristic  shape,  and  each 
one  as  a  rule  formed  by  the  activity  of  the  living  protoplasm 
within  the  compass  of  a  single  cell. 

Whilst  many  unicellular  organisms  and  many  sponges  thus 
manufacture  for  themselves  skeletons  of  a  siliceous  character, 
others,  living  perhaps  in  the  same  water,  secrete  skeletons  which 
are  composed  of  carbonate  of  lime.  Such,  for  example,  are  the 
Forarninifera,  the  dead  calcareous  shells  of  which  (Fig.  4) 
accumulate  to-day  at  the  bottom  of  the  deep  sea  in  many  places 
in  the  form  of  ooze,  while  in  the  Cretaceous  period  of  the  earth's 
history  they  played  the  principal  part  in  building  up  the  chalk 
cliffs  on  the  south  coast  of  England.  Each  of  these  microscopic 
shells,  which  are  often  of  extreme  beauty  and  frequently 
ornamented  with  elaborate  patterns,  is  formed  as  a  Drotective 
envelope  by  a  simple  protoplasmic  organism  closely  resembling 
an  Amoeba. 

It  is  evident,  then,  that  we  must  attribute  to  living  proto- 
plasm a  very  remarkable  power  of  selection  or  choice,  for  it  is 
able,  as  it  were,  to  pick  out  certain  materials  from  its  environ- 
ment for  its  own  purposes  and  to  reject  others.  We  have  already 
seen  this  in  the  case  of  the  selection  of  food  materials  by  the 
Amoeba ;  we  see  it  again  in  the  selection  of  the  materials  for 
skeleton  formation  in  other  simple  organisms.  The  fact  that 
one  organism  will  select  silica  while  another  selects  carbonate  of 
lime  from  the  same  sample  of  sea  water  and  for  the  same  purpose, 
must  correspond  to  some  deep-seated  difference  in  the  protoplasm 
of  which  they  are  composed,  and  illustrates  very  well  the  diverse 
potentialities  of  this  remarkable  substance. 


CHAPTER  III 

Haematococcus — The  differences  between  animals  and  plants. 

IN   striking   contrast    to   Amoeba,   which,   though  primitive, 

is  nevertheless  a  very  ^ncaT~exampIe  of  an  animal  organism, 
stands  Hsematococcus  or  Jiphserella,  which,  by  botanists  at  any 
rate,  is  regarded  as  a  very  simply  organized  member  of  the 
vegetable  kingdom.  Like  Amoeba,  this  organism  is  of  microscopic 
size,  consisting  of  only  a  single  cell  or  protoplasmic  unit. 

Haematococcus  pluvialis  (also  known  as  Sphcerella  lacustris) 
occurs  in  temporary  pools  of  stagnant  rain-water  or,  in  the  rest- 
ing condition,  in  dried-up  mud  or  dust.  Though  individually 
invisible,  or  barely  visible,  to  the  naked  eye,  it  may  occur  in  such 
dense  associations  as  to  give  the  water  a  bright  red  (or  some- 
times green)  colour  and  form  a  red  crust  on  the  sides  of  the 
vessel  in  which  it  is  cultivated.  A  closely  related,  if  not  identical, 
species  (Hcematococcus  nivalis)  is  the  cause  of  the  red  patches 
which  are  sometimes  observed  on  the  snow-fields  in  Arctic 
regions.  Cultivation  is  easy,  and  the  same  stock  may  be  kept 
going  for  many  years  and  multiplied  indefinitely.  Twenty 
years  ago  or  more  I  had  a  sample  given  to  me  in  Australia, 
descendants  of  which  are  now  flourishing  in  full  vigour  in 
England.  It  can  be  dried  up  when  not  required  and  when 
wanted  again  in  the  active  condition  needs  only  to  be  supplied 
with  fresh  rain-water  and  placed  in  the  sun. 

In  the  resting  condition  each  individual  consists  of  a  spherical 
protoplasmic  body  (Fig.  5,  A)  of  a  bright  red  or  green  colour,  or 
sometimes  partly  green  and  partly  red,  with  a  more  or  less 
centrally  placed  nucleus  (nu.).  It  differs  from  an  Amoeba  in  the 
presence  of  a  very  distinct,  firm  cell-wall  (c.w.)  on  the  outside,  as 
well  as  in  its  definite  shape  and  characteristic  colour.  The  cell- 
wall  is  composed  of  cellulose,  one  of  a  group  of  chemical  com- 
pounds known  as  carbohydrates.  These  compounds  are  all 
characterized  by  the  fact  that  they  contain  only  three  elements, 
carbon,  hydrogen  and  oxygen,  the  hydrogen  and  oxygen  occurring 


28 

V 


OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


in  the  same  proportions  as  in  water  (H20).  Thus  the  chemical 
formula  for  cellulose  is  (C6Hi005)n,  while  that  for  glucose  or  grape 
sugar,  another  carbohydrate,  is  C6Hi206.  The  presence  of  very 
definite  cell-walls,  composed  of  cellulose  and  formed  as  a  secretion 
by  the  living  protoplasm,  is  very  characteristic  of  vegetable  as 
contrasted  with  animal  organisms. 

The  protoplasm  which  lies  inside  the  cell-wall  is,  as  we  have 
already  said,  either  red,   green   or  parti-coloured.     The  green 


FIG.  5. — Structure  and  Life-history  of  Hcematococcus  plnvialis. 

A.  Resting  stage  with  thick  cell-wall. 

B.  Division  into  four  motile  zoospores  within  the  old  cell-wall. 

C.  Free-swimming  zoospore. 

D.  Division  of  the  resting  cell  into  32  microzooids  or  gametes. 

E.  Free-swimming  gamete. 

F — G.  Conjugation  of  two  gametes.* 
H.  Zygote  with  four  flagella,  formed  by  conjugation. 
J.  Zygote  with  flagella  withdrawn. 

K.  Resting  cell  formed  from  the  zygote  by  secretion  of  a  thick  cell-wall. 

c.w.  cell-wall ;  fl.  flagellum ;  nu.  nucleus ;  py.  pyrenoid ;  vac.  vacuole. 

(Figs.  D— K  adapted  from  Peebles.) 

colour  is  due  to  the  presence  of  that  extremely  characteristic 
vegetable  pigment  known  as  chlorophyll,  a  remarkable  pro- 
duct of  the  activity  of  the  living  protoplasm  with  which  we  are 
all  familiar  in  the  case  of  ordinary  green  plants.  The  red 
pigment,  known  as  haematochrome,  is  but  a  slight  chemical 
modification  of  the  green  chlorophyll,  and  the  one  may  readily 
be  converted  into  the  other.  If  some  nitrogenous  substance, 
such  as  a  small  quantity  of  manure  water,  be  placed  in  a 
vessel  containing  red  Hsematococcus,  the  latter  will  in  a  short 
time  assume  a  bright  green  colour,  whence  we  may  conclude  that 
the  red  colouration  is  probably  an  effect  of  nitrogen  starvation. 


H^MATOCOCCUS  20 

When  a  dried -up  sample  of  Hsematococcus  is  supplied  with 
fresh  rain-water  and  placed  in  the  sunlight  it  undergoes  a 
remarkable  change.  The  protoplasmic  body  within  the  cell-wall 
undergoes  division,  first  into  two  and  then  into  four  parts 
(Fig.  5,  B).  This  is  effected  by  a  process  of  simple  fission  exactly 
comparable  to  that  which  we  have  already  described  in  the  case 
of  Amoeba.  The  four  parts  or  daughter  cells  (sometimes  called 
zoospores)  are  for  a  short  time  kept  together  within  the  old  cell- 
wall,  but  presently  this  ruptures  and  they  escape  into  the 
surrounding  water. 

It  will  now  be  seen  that  these  so-called  zoospores  differ  greatly 
in  structure  from  the  resting  Haematococcus.  Instead  of  being 
spherical  they  are  more  or  less  oval  or  pear-shaped  in  outline 
(Fig.  5,  C).  Each  has  secreted  a  new  cellulose  wall  of  its  own 
(c.w.},  but  this  is  separated  from  the  main  protoplasmic  body  by 
a  considerable  space,  or  vacuole,  filled  with  water  (vac.\  across 
which  stretch  delicate  threads  of  colourless  protoplasm,  which 
keep  the  protoplasmic  body  in  position.  The  main  mass  of 
protoplasm  is  coloured  red  or  green,  as  before,  and  contains  the 
nucleus  (nu.).  At  one  end  it  is  drawn  out  into  a  kind  of  beak, 
from  which  two  very  long  and  slender  threads  of  colourless 
protoplasm  (ft.)  pass  outwards,  through  minute  apertures  in  the 
cellulose  wall,  into  the  surrounding  water.  Owing  to  their 
whip-like  form  and  characteristic  lashing  movements,  these  are 
termed  flagella. 

It  is  by  the  very  rapid  movements  of  the  flagella  that  the 
locomotion  of  the  active  Haematococcus  is  effected.  They  are 
carried  in  front,  and  the  body  of  the  organism  is  pulled  through 
the  water  by  their  action  much  as  a  boat  is  pulled  by  a  pair  of 
sculls,  at  a  rate  which,  though  very  slow  when  judged  by  our 
own  standards,  appears  very  rapid  when  considered  in  relation  to 
the  minute  size  of  the  organism.  The  movements  of  the  flagella 
are  somewhat  complex  and  of  an  undulatory  kind.  They  appear 
to  be  entirely  automatic,  but  it  seems  probable  that  they  must 
be  performed  in  response  to  stimuli  which  we  are  unable  to 
recognize.  The  flagella  are  much  more  definite  and  highly 
specialized  structures  than  the  pseudopodia  of  an  Amoeba.  Like 
the  latter,  however,  they  owe  their  value  as  organs  of  locomo- 
tion to  that  inherent  power  of  contraction  which  is  one  of  the 
most  characteristic  features  of  living  protoplasm.  It  is  probable 
that  each  really  consists  of  several  very  slender  filaments,  lying 


30          OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

side  by  side,  and  that  the  complex  undulatory  movements  are 
due  to  alternating  contractions  and  relaxations  of  these. 

The  presence  of  a  firm  cell-wall  makes  the  protrusion  of 
pseudopodia  in  the  case  of  Haematococcus  quite  impossible,  and 
at  the  same  time  prevents  the  organism  from  taking  in  any 
solid  food,  for  there  is  no  aperture  through  which  such  food 
could  pass.  It  must  therefore  depend  entirely  for  its  food  supply 
upon  substances  which  are  able  to  pass  through  the  cell-wall 
in  a  state  of  solution.  These  substances  are  all  very  simple 
chemical  compounds,  consisting  of  certain  mineral  salts  and 
carbon  dioxide  gas,  which,  amongst  them,  contain  all  the  elements 
necessary  for  the  formation  of  protoplasm.  They  are,  however, 
very  stable  bodies,  with  little  or  no  affinity  for  oxygen  gas  and 
little  potential  energy.  They  cannot,  therefore,  by  themselves 
supply  the  energy  which  the  organism  requires  for  its  vital 
activities.  Energy  has  to  be  supplied  from  the  environment 
and  the  simple  food  materials  thereby  partially  deoxidised  and 
combined  together  in  more  complex  and  less  stable  molecules 
containing  stores  of  potential  energy  which  can  be  liberated  by 
oxidation  as  required. 

The  energy  which  Haematococcus  uses  for  the  building  up  of 
its  complex  molecules  is,  as  we  have  already  observed  for  green 
plants  in  general,  the  radiant  energy  of  the  sun,  in  the  form  of 
light.  The  process  is  known  as  photosynthesis,  and  can  only 
take  place  in  organisms  which  possess  chlorophyll  or  some 
functionally  equivalent  pigment,  such  as  haematochrome.  In 
some  way  or  other  the  pigment  absorbs  the  energy  of  the  light 
rays  and  renders  it  available  for  the  process  of  constructive 
metabolism  (which  in  plants  is  also  spoken  of  as  assimilation). 

The 'first  step  in  this  complex  process  involves  alchemical 
decomposition,  carbon  dioxide,  obtained  in  solution  from  the 
surrounding  water,  being  decomposed  with  evolution  of  free 
oxygen  gas. 

We  have  already  seen  that  in  the  combustion  of  charcoal 
the  reverse  of  this  takes  place,  carbon  and  oxygen  combining 
to  form  carbon  dioxide  and  the  act  of  combination  being  accom- 
panied by  the  liberation  of  energy.  It  is  obvious  that  if  energy  is 
liberated  in  the  one  process  a  corresponding  amount  must  be 
absorbed  in  the  other. 

When  a  glass  jar  of  water  containing  Haematococcus,  or  any 
green  aquatic  plant,  is  placed  in  bright  sunlight  the  decomposition 


PHOTOSYNTHESIS  31 

of  carbon  dioxide  in  the  plant  takes  place  so  rapidly  that 
minute  bubbles  of  oxygen  may  often  be  seen  rising  to  the  surface 
of  the  water.  If,  for  example,  we  cut  off  a  leafy  branch  of  the 
common  Canadian  water  weed,  known  as  Elodea  canadensis,  and 
fix  it  under  water  in  such  a  jar,  it  is  possible  to  arrange  the 
experiment  so  that  a  regular  stream  of  small  oxygen  bubbles  will 
be  given  off  from  the  cut  end,  and  it  is  further  possible  to  adjust 
the  experiment  so  delicately  that  the  interposition  of  a  dark 
screen  between  the  jar  and  the  sunlight  will  cause  the  immediate 
cessation  of  the  stream  of  bubbles,  which  will  start  again  the 
instant  the  screen  is  removed.  This  simple  and  beautiful 
experiment  clearly  demonstrates  the  dependence  of  the  process 
of  decomposition  of  carbon  dioxide  upon  the  presence  of 
sunlight. 

The  oxygen  liberated  in  this  way  is  not  (with  the  exception  of 
a  relatively  small  quantity  used  in  respiration)  required  by  the 
organism,  and  is  accordingly  at  once  discharged  into  the  sur- 
rounding medium.  The  carbon,  on  the  other  hand,  is  needed  for 
the  manufacture  of  new  protoplasm.  It  is  never  actually  set  free 
as  carbon,  but  its  molecules  are  probably  recombined — under  the 
influence  of  the  sunlight— with  the  molecules  of  water  to  form  the 
carbohydrate  known  as  glucose  or  grape  sugar.  This  process 
may  be  represented  by  the  equation — 

6  C02  +6  H20     =       6  02       +      C6H1206 

(Carbon  Dioxide)  (Water)          (Oxygen)          (Glucose). 

It  is  probable  that  a  simpler  compound,  possibly  formaldehyde 
(CH20),  is  formed  as  an  intermediate  product,  while,  on  the  other 
hand,  the  glucose  appears  to  be  rapidly  converted  into  starch, 
which  is  the  first  visible  product  of  the  process  of  photosynthesis 
in  the  plant  cell. 

Starch,  like  glucose  and  cellulose,  is  a  carbohydrate,  and, 
though  differing  in  many  of  its  chemical  and  physical  properties, 
has  the  same  general  formula  as  the  latter  (C6Hi005)n.  This 
means  simply  that  the  elements  carbon,  hydrogen  and  oxygen 
are  present  in  the  same  proportions  as  in  cellulose,  but  they  must 
be  linked  together  differently  in  the  molecule. 

The  first  step  in  the  actual  construction  of  the  proteid  molecule 
is  then  the  combination  of  carbon  with  the  elements  hydrogen 
and  oxygen  to  form  a  carbohydrate.  In  the  higher  plants  starch 
first  appears  in  the  chlorophyll-containing  cells  of  the  leaves  in 


32         OUTLINES   OF   EVOLUTIONARY  BIOLC&Y 

the  form  of  starch  grains,  which  may  afterwards  be  converted 
into  sugar  again  and  then,  in  solution,  transferred  to  other  parts 
of  the  plant,  where  it  is  redeposited  and  stored  up,  once  more  in  the 
form  of  starch  grains,  for  future  use,  as  in  the  potato  and  in  starch- 
containing  seeds  such  as  peas  and  beans. 

Both  starch  and  chlorophyll  are,  at  any  rate  usually,  formed  in 
the  cell  in  connection  with  specialized  portions  of  the  protoplasm 
known  as  plastids.  These  are  regarded  as  living  bodies  which 
are  specially  concerned  in  the  formation  of  chlorophyll,  starch  and 
other  substances.  When  they  contain  chlorophyll  they  are  termed 
chloroplastids,  and  in  the  higher  plants  they  r  take  the  form  of 
numerous  minute  "  chlorophyll  corpuscles "  of  definite  shape, 
which  occur  in  abundance  in  the  cells  of  all  green  parts,  and  in 
connection  with  which  the  starch  grains  are  formed  (vide  Fig.  26). 
In  Haematococcus  practically  the  whole  central  mass  of  cytoplasm 
is  coloured  by  the  chlorophyll  (or  haematochrome)  and  may 
perhaps  be  regarded  as  a  single  large  chloroplastid.  The  starch, 
however,  is  collected  around  small,  specialized,  proteid  bodies 
imbedded  in  the  general  mass  of  cytoplasm.  These  are  known 
as  pyrenoids  (Fig.  5,  C,  py.). 

We  have  thus  seen  how  the  green  plant  obtains  the  carbon, 
hydrogen  and  oxygen  which  it  requires  for  the  manufacture  of 
protoplasm.  Other  elements,  however,  have  to  be  combined  with 
the  molecules  of  carbohydrate  before  proteids  can  be  formed.  These 
are  nitrogen,  sulphur  and,  sometimes  at  any  rate,  phosphorus,  all 
of  which  are  obtained  by  green  plants  by  the  decomposition  of 
mineral  salts — nitrates,  phosphates  and  sulphates — which  exist  in 
solution  in  the  water  or  damp  soil  in  which  the  plant  grows.  In 
the  higher  plants  these  substances  are  taken  up  by  the  root-hairs 
and  transmitted  to  the  leaves  by  a  system  of  vessels  and  tracheids 
analogous  to  the  circulatory  system  of  animals.  In  such  a  plant 
as  Haematococcus  they  simply  diffuse  into  the  protoplasmic  body 
from  the  surrounding  water  by  the  process  of  osmosis.  Exactly 
what  happens  when  they  meet  with  the  carbohydrates  we  do  not 
know,  but  further  chemical  combinations  must  take  place 
under  the  influence  of  sunlight,  which  finally  result  in  the 
formation  of  new  proteid  molecules  which  are  added  to,  the 
already  existing  protoplasm. 

Kespiration  in  Haematococcus  probably  takes  place  exactly 
as  in  Amoeba,  but  it  is  more  easily  studied  in  the  case  of  the 
higher  plants.  In  daylight  the  process  is  greatly  obscured  by 


CONJUGATION   OF   GAMETES  33 

the  absorption  of  carbon  dioxide  and  the  evolution  of  oxygen  gas 
which  accompany  photosynthesis.  In  darkness,  however,  the 
gaseous  interchange  which  forms  the  essential  feature  of  respira- 
tion can  readily  be  detected,  carbon  dioxide,  produced  by  oxidation 
of  the  protoplasm,  being  given  off  and  oxygen  taken  in. 

No  special  organ  of  excretion  has  been  observed  in  Haemato- 
coccus,  though  a  contractile  vacuole  occurs  in  closely  allied  forms, 
and  waste  products  must  simply  diffuse  into  the  surrounding 
water  through  the  permeable  cell- wall. 

We  have  already  seen  that  Haematococcus  multiplies  itself  by 
simple  fission  within  the  old  cell-wall.  This  process  usually 
results  immediately  in  the  production  of  four  new  individuals. 
Under  favourable  circumstances  it  may  be  repeated  very  rapidly, 
without  the  organism  going  through  any  true  resting  stage,  so 
that  in  a  short  space  of  time  the  number  of  active  zoospores 
may  be  very  largely  increased.  The  individuals  thus  produced 
are  usually  all  of  the  same  form,  and  ultimately  of  the  same  size, 
as  the  parent.  Occasionally,  however,  a  somewhat  different 
process  of  multiplication  takes  place.  Instead  of  dividing  into 
four  relatively  large  zoospores  a  resting  individual  may  divide  into 
thirty-two  or  sixty-four  much  smaller  "microzooids"  (Fig.  5,  D), 
which  differ  from  the  ordinary  active  form  in  the  absence  of  the 
characteristic  cell- wall  with  its  underlying  vacuole. 

The  microzooids  (Fig.  5,  E)  swim  actively  about  by  means  ol 
their  flagella.  Sooner  or  later,  however,  they  come  together  in 
pairs  (Fig.  5,  F,  G),  and  the  members  of  each  pair  fuse  completely 
with  one  another  to  form  a  single  individual  (Fig.  5,  H)  with 
four  flagella,  which  presently  loses  its  flagella,  secretes  around 
itself  a  thick  cell-wall,  and  enters  upon  the  resting  state 
(Fig.  5,  J,  K).  From  this  resting  individual  new  generations  will 
be  produced  by  the  ordinary  method  of  division  into  zoospores. 

We  have  here  an  excellent  illustration  of  what  is  usually 
termed  sexual  reproduction,  the  essential  feature  of  which  is  the 
union  or  conjugation  of  two  sexual  cells  or  gametes  (in  this  case 
the  microzooids)  to  form  a  single  cell,  the  zygote,  which  is  the 
starting  point  of  a  fresh  series  of  cell  generations.  This  important 
process  will  be  discussed  more  fully  in  a  subsequent  chapter. 

We  have  spoken  of  Amoeba  as  an  animal,  and,  as  we  have  seen, 
many  people  regard  Haematococcus  as  a  plant.  We  must  next 
endeavour  to  find  out  what  it  is  that  really  differentiates  a  plant 
from  an  animal.  Of  course  amongst  Jdie  more  highly  organized 

B.  .  y> 


34         OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

members  of  the  animal  and  vegetable  kingdoms  we  can  point  to 
many  obvious  distinctions.  The  higher  plants  are  fixed  and 
stationary,  while  the  animals  move  about  from  place  to  place  by 
means  of  special  organs  of  locomotion.  The  animals  have 
complex  digestive,  respiratory,  excretory,  nervous  and  sensory 
organs,  which  are  wanting  in  the  plants.  Lastly,  the  animals 
have  no  chlorophyll  and  cannot  therefore,  like  the  green  plants, 
obtain  their  supplies  of  energy  directly  from  the  sun's  rays  by 
photosynthesis,  but  must  depend  upon  the  potential  energy  con- 
tained in  the  complex  molecules  of  their  food,  which  they  obtain 
ready  made  from  the  bodies  of  other  organisms. 

Amongst  the  lower  organisms,  however,  we  find  that  most  of 
these  distinctions  disappear.  Thus  many  of  the  lower  plants 
move  about  actively  while  many  of  the  lower  animals,  such  as 
the  sponges,  hydroids  and  corals,  are  fixed  and  stationary  in  the 
adult  condition,  though  still  showing  their  animal  nature  in  other 
respects,  such  as  their  method  of  nutrition.  This  mixture  of 
what  were  once  regarded  as  distinctively  animal  and  vegetable 
characters  in  such  forms  as  the  corals  and  hydroids  gave  rise  to 
the  name  "  zoophytes,"  or  animal-plants,  by  which  these  organisms 
were  known  to  the  older  naturalists. 

When  we  descend  to  forms  still  lower  in  the  scale  of  organiza- 
tion, consisting  each  of  a  single  cell,  we  find  that  every  dis- 
tinction may  disappear  except  that  of  the  presence  or  absence  of 
chlorophyll  and  the  mode  of  nutrition  immediately  dependent 
thereon : — plant-like  or  holophytic  as  in  Hsematococcus,  animal- 
like  or  holozoic  as  in  Amoeba.1 

It  cannot  be  maintained,  however,  that  even  these  characters 
form  an  absolute  distinction  between  plants  and  animals,  for,  in 
the  first  place,  many  undoubted  plants,  such  as  the  Fungi,  have 
lost  their  chlorophyll  by  degeneration,  and,  in  the  second  place, 
while  botanists  claim  Haematococcus  and  the  forms  closely  related 
to  it  as  plants,  zoologists  claim  them  as  animals,  chiefly  because 
they  are  so  closely  related  in  structure  to  other  unicellular 
flagellates  which  contain  no  chlorophyll  that  we  cannot  refuse  to 
include  them  in  the  same  group. 

1  The  nutrition  of  any  typical  green  plant  is  holophytic,  that  of  any  typical 
animal  holozoic.  The  latter  term  implies  the  taking  in  of  solid  food  derived  from 
the  bodies  of  other  organisms,  and  is  thus  distinguished  from  the  saprophytic  type 
of  nutrition  met  with  in  many  of  the  lower  animals  and  plants  (e.g.  Fungi),  which 
consists  in  the  absorption  of  liquid  food  derived  from  the  decaying  bodies  of  other 
organisms. 


PLANTS   AND  ANIMALS  35 

The  explanation  of  the  difficulty  really  lies  in  the  fact  that  I 
both  plants  and  animals  originally  sprang  from  common  unicel- 
lular ancestors  which  were  neither  the  one  thing  nor  the  other. 
The  first  appearance  of  chlorophyll  initiated  the  great  cleavage 
between  the  animal  and  vegetable  kingdoms.  Thenceforward 
the  two  great  groups  developed  each  along  lines  of  its  own.  By 
virtue  of  their  chlorophyll  the  green  plants  became  the  great 
proteid  manufacturers  of  the  world,  and  the  animals  became 
dependent  upon  them  for  their  food  supply.  Animals  are  largely 
dependent  upon  green  plants  in  another  respect  also,  for  the 
latter,  as  we  have  seen,  split  up  the  carbon  dioxide,  formed  as  a 
waste  product  in  the  respiration  of  both  groups,  and  thus  set  free 
fresh  supplies  of  the  necessary  oxygen. 

Closely  correlated  with  the  differences  in  their  mode  of  nutri- 
tion are  the  great  differences  in  the  mode  of  life  of  the  higher 
plants  and  animals.  Plants  have  no  need  to  move  from  place  to 
place  in  search  of  food,  which  they  obtain  from  the  air  and  the 
soil,  and  the  supplies  of  which  are  constantly  renewed  by  wind 
and  rain.  Highly  organized  animals,  on  the  other  hand,  would 
soon  exhaust  their  supplies  if  they  remained  always  in  the  same 
place,  and  it  is  doubtless  the  necessity  for  actively  seeking  out 
and  even  contending  one  with  another  for  fresh  supplies  that 
has  brought  about  the  wonderful  elaboration  and  perfection  of 
their  organization,  while  the  fact  that  in  so  many  cases  they 
devour  one  another,  instead  of  remaining  directly  dependent  upon 
vegetable  organisms,  must  have  greatly  intensified  the  struggle 
for  existence  and  correspondingly  increased  the  rate  of  progress 
in  their  evolution. 


D   2 


CHAPTER  IV 

The  cell  theory — Unicellular  organisms — Differentiation  and  division  of 
labour — Co-operation — The  transition  from  the  unicellular  to  the 
multicellular  condition — The  early  development  of  multicellular  animals 
and  plants. 

WE  have  seen  that,  although  Hsematococcus  and  Amoeba  differ 
widely  from  one  another  in  various  respects,  they  nevertheless 
exhibit  a  fundamental  agreement  in  structure,  for  each  consists 
essentially  of  a  single  nucleated  mass  of  protoplasm,  each  is  a 
single  cell. 

The  history  of  the  term  cell  is  a  curious  one,  and  affords  a 
good  illustration  of  the  manner  in  which  our  scientific  conceptions 


picf.  6. — Thin  Section  of  a  Bottle  Cork,  showing  cellular  Structure,  X  170. 
(From  a  photograph.) 

gradually  become  modified  and  improved  as  our  knowledge 
increases.  Nothing  was  known  of  cells  before  the  invention  of 
the  microscope,  but  in  the  latter  half  of  the  seventeenth  century 
this  invention  o/pened  up  an  entirely  new  field  of  research,  and 
enabled  the  earlier  microscopists  to  lay  the  foundations  of  the 
modern  science  of  biology.  To  Eobert  Hooke  has  been  assigned  the 
credit  of  fir/si  observing  the  cellular  structure  of  vegetable  tissues, 
and  his  observations,  published  in  1665,  were  soon  afterwards 


OKIGIN   OF   THE   TERM  CELL 


37 


confirmed  by  Nehemiah  Grew,  who  published  his  great  work  on 
;he  Anatomy  of  Plants  in  1672. 

It  is  now  a  matter  of  common  knowledge  that  many  vegetable 
;issues,  such  as  cork  and  pith,  when  seen  in  section  under  the 
microscope,  exhibit  a  honeycomb-like  appearance,  being  composed 


CortH 


FIG.  7.— Part  of  a  cross  Section  of  a  Maize  Boot,  showing  cellular 

Structure,  X  84.     (From  a  photograph.) 

Cort.,  cortex;  pi.,  pith;  V.,  vessels. 

of  rectangular  or  polygonal,  or  it  may  be  spherical  chambers 
separated  from  one  other  by  firm  walls.  This  structure  is  very 
well  shown  in  Fig.  6,  which  represents  part  of  a  thin  section  of  an 
ordinary  bottle  cork,  and  in  Fig.  7,  which  represents  part  of  a 
transverse  section  of  a  root  of  maize.  It  was  the  resemblance  to 
a  honeycomb  that  led  to  the  application  of  the  term  cell  to 
these  chambers.  The  earlier  observers  naturally  attached  most 


38    OUTLINES  OF  EVOLUTIONAEY  BIOLOGY 

importance  to  the  cell-walls,  which  indeed  are  alone  visible  in 
dead  tissues  such  as  dry  cork  and  pith. 

It  was   not  until  1846  that  von  Mohl  first  gave  the  name 

protoplasm  to  the  slimy  contents  of  the  cells  in  living  tissues. 
It  then  gradually  became  evident  that  this  protoplasm  was  the 
really  vital  constituent  of  the  cell,  and  that  it  was  identical  in 
nature  with  the  substance  of  which  minute  naked  organisms 
such  as  the  Amoeba  are  composed,  and  which  was  already 
known  by  the  name  sarcode,  given  to  it  by  Dujardin  in 
1835. 

The  cell  theory,  first  propounded  in  a  very  imperfect  form  by 
Schleiden  and  Schwann,  about  the  year  1838,  rapidly  developed, 
during  the  course  of  the  nineteenth  century,  into  one  of  the  most 
fertile  generalizations  of  natural  science.  At  the  present  day  the 
term  cell  is  extended  to  protoplasmic  units  which  may  have  no 
cell-walls  at  all,  and  to  which  therefore  it  is  etyrnologically  quite 
inapplicable,  and  for  a  long  time  a  cell  has  been  defined  simply 
as  a  single  nucleated  mass  of  protoplasm. 

In  accordance  with  the  cell  theory  such  nucleated  masses  of 
protoplasm  are  the  organic  units  of  which  the  bodies  of  all  living 
things  are  built  up.  The  simpler  organisms,  such  as  Amoeba 
and  Haematococcus,  consist  each  of  a  single  unit  only,  and  are 
therefore  said  to  be  unicellular,  while  the  more  complex  forms, 
both  of  animals  and  plants,  consist  each  of  many  such  units  united 
together  in  a  multicellular  body.  Moreover,  we  now  know  that  cells 
never  originate  de  novo  but  multiply  by  division,  so  that  each  one 
is  the  immediate  descendant  of  a  pre-existing  cell,  a  very 
important  fact  which  was  emphasized  by  Virchow  in  his  often 
quoted  phrase  "Omnis  cellula  e  cellula." 

The  zoologist  includes  under  the  name  Protozoa  all  those - 
unicellular  organisms  which  he  claims  as  members  of  the  animal  - 
kingdom,  whilst  the  unicellular  plants  are  relegated  to  the  domain  - 
of  the  botanist  under  the  name  Projtophyta,  but,  as  we  have  - 
already  seen,  it  is  impossible  to  draw  a  rational  line  of  demarcation  - 
between  these  two  groups  and  they  are  often  included  together - 
under  Haeckel's  term  Protista. 

I      The  outstanding  feafure  in  all  these  simple  forms  of  life  is  tha(5  •*• 
i  the  single  cell  is  a  complete  and  self-supporting  organism.     Ifc— 
has  to  perform  all  the  necessary  vital  functions  for  itself,  by  means 
of  such   simple  organs,  temporary  or   permanent,   as   can   he- 
produced  by  differentiation  within  the  microscopic  limits  of  its 


PABAMGECIUM 


39 


protoplasmic  body.1  It  is  surprising  what  a  high  degree  of 
organization,  as  indicated  by  complexity  of  structure,  may  be 
attained  in  such  a  case. 

r  ^Differentiation  and  division  of  labour  are  the  results  of  progressive 
evolution,  and  at  the  same  time  the  means  by  which  further 
progress  is  effected.  Even  in  Amoeba  and  Haematococcus  we  see 
clearly  enough  the  operation  of  these  two  great  principles.  Many 
unicellular  organisms,  however,  exhibit  a  far  higher  degree  of 
organization.  The  Protozoan,  Paramoecium  (Fig.  8),  so  common 
in  infusions  of  decaying  vegetable  matter,  swims  actively  about  by 
means  of  innumerable  short  vibratile  cilia  which  project  all  over 
the  surface  of  the  body.  It  has  a  definite  mouth,  through  which 


PV 


FIG.  8. — Paramcecium  aurelia,  X  300.     (From  Marshall  and  Hurst's 
"Practical  Zoology.") 

AV,  anterior  contractile  vacuole  (dilated) ;  EC,  ectoplasm  with  trichocysts ;  EN,  endo- 
plasm ;  EP,  micronucleus  ;  FV,  food  vacuole ;  M,  mouth ;  MY,  contractile  fibrillae  ; 
N,  meganucleus ;  OG-,  groove  leading  to  mouth :  PV,  posterior  contractile  vacuole 
(contracted) ;  TR,  discharged  trichocyst  threads ;  X,  cilia. 

solid  food  particles  are  taken  in,  and  a  less  definite  anal  spot  at 
which  fcecal  matter  is  ejected.  It  has  special  weapons  of  offence  or 
defence  (trichocysts)  which  can  shoot  out  from  the  surface  of  the 
body  long  threads  when  the  animal  is  irritated.  It  has  two 
contractile  vacuoles,  each  with  a  system  of  radiating  canals 
discharging  into  it,  and  it  has  two  nuclei,  large  and  small 
(meganucleus  and  micronucleus),  which  appear  to  fulfil  different 
functions  and  each  of  which  doubtless  has  a  complex  structure  of 
its  own.  How  complex  the  structure  of  the  nucleus  may  be  we 
shall  be  better  able  to  judge  when  we  come  to  speak  of  the 
phenomena  of  nuclear  division  in  a  subsequent  chapter. 

1  The  organs  into  which  a  single  cell  may  be  differentiated  are  sometimes  spoken 
of  as  organellae,  but  if  we  define  an  organ  as  any  part  of  an  organism  which  is 
specialized  for  the  fulfilment  of  some  particular  function,  it  is  quite  unnecessary  to 
distinguish  the  organs  of  a  single  cell  by  a  special  term. 


\ 


40          OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Paramcecium  and  many  other  Protozoa,  again,  have  specialized 
contractile  threads  of  protoplasm  lying  just  beneath  the  surface 
of  the  body,  comparable  to  the  muscle  fibres  of  higher  animals, 
which  enable  them  to  perform  movements  of  a  different  kind 
from  those  effected  by  flagella  or  cilia.  By  means  of  such  con- 
tractile fibres,  localized  mainly  in  a  long  stalk,  the  bell- 
animalcule,  Yorticella  (Fig.  9,  10),  can  instantaneously  draw  in 
its  ciliated  disc  and  pull  itself  out  of  harm's  way  on  the  approach 
of  danger."  Other  examples  of  these  highly  organized  ciliate 
Protozoa  (Infusoria)  are  shown  in  Fig.  9.  These  forms  should 
be  cjfcpared  with  the  Radiolaria  and  Foraminifera  represented  in 
Figs.  3  and  4. 

Another  very  important  factor  in  the  progress  of  organic 
evolution  is  supplied  by  the  principle  of  co-oDemtion  between 
different  organic  units.  This  is  illustrated  to  some  extent  in  the 
process  known  as  colony-formation  met  with  in  certain 
Protozoa  and  Protophyta.  Carchesium,  Epistylis  and  Zootham- 
nium  (Fig.  9,  11—15),  for  example,  are  colony-forming  Infusoria 
closely  related  to  Yorticella.  Yorticella  itself  multiplies  rapidly 
by  simple  longitudinal  fission.  The  bell-shaped  protoplasmic 
body  at  the  end  of  the  stalk  divides  into  two  parts,  one  of  which 
swims  away,  attaches  itself  to  some  foreign  object,  and  develops 
a  new  stalk.  If,  instead  of  separating,  the  two  daughter  cells 
remained  together  and  went  on  dividing,  and  if  this  division  also 
extended  each  time  to  the  upper  part  ofthe  stalk,  the  organism 
would  presently  arrive  at  a  branching,  tree-like  condition.  This 
is  what  has  happened  in  Carchesium,  Epistylis  and  Zootham- 
nium,  and  also  in  various  other  Protozoa. 

This  arborescent  type  of  colony-formation,  moreover,  is  by 
no  means  the  only  one  met  with  amongst  the  Protista.  It  is 
characteristic  of  stalked  forms.  Other  forms,  which  are  not 
stalked,  may  give  rise  to  free-swimming  or  floating,  solid  or 
hollow  aggregates  of  plate-like  or  it  may  be  spherical  shape. 
The  development  of  such  colonies  is  perhaps  nowhere  better 
seen  than  in  the  small  group  of  unicellular  organisms  to  which 
Hsematococcus  belongs,  the  members  of  which  are  known,  on 
account  of  their  plant-like  character  and  their  possession  of 
flagella,  as  Phytoflagellata. 

Hsematococcus,  or  the  closely  related  Chlamydomonas,  may  be 
taken  as  the  starting  point  of  the  series.  These  two  are  always 
solitary,  the  individuals  separating  completely  from  one  another 


CILIATE  PROTOZOA 


41 


K   A 

\ 


;r 


Fia.  9.— Various  forms  of  Ciliate  Protozoa  (Infusoria),  highly  magnifiea 
(From  Haeckel's  "  Kunstformen  der  Natur.") 

1,  Codonella ;  2,  3,  Dictyocysta  (shell  only);  4,  Tintinnopsis  (shell  only);  5,  Cyttaro 
cyclis  (shell  only);  6,  Petalotricha  (shell  only);  7,  8,  Stentor ;  9,  Freia ;  10,  For 
t'icella;  11,  12,  Carchetium ;  13,  Epistylis ;  14,  15,  Zoothamnium. 


42 


OUTLINES   OF   EVOLUTIONAKY  BIOLOGY 


after  each  cell-division.  In  Pandorina  (Fig.  10),  however,  which 
is  another  common  fresh-wafer  organism,  the  individuals  pro- 
duced by  fission  remain  together  and  form  a  solid  ball,  composed 
of  from  sixteen  to  sixty-four  cells,  enclosed  in  a  gelatinous 
envelope.  These  little  colonies  swim  about  actively  by  means  of 
their  flagella,  which  project  from  the  surface  in  pairs,  one  pair 
belonging  to  each  individual.  Multiplication  is  usually  effected 
by  the  division  of  each  individual  cell  into  sixteen,  so  that  as 
many  daughter  colonies  are  formed  as  there  were  cells  in  the 
parent  colony,  and  these  daughter  colonies  finally  separate 

from  one  another.  Eudorina 
(Fig.  40)  forms  slightly"  larger 
colonies  of  a  similar  kind,  but 
with  the  component  individuals 
somewhat  widely  separated  from 
one  another  by  the  gelatinous 
matrix.  Finally,  in  Volvox 
(Fig.  11),  one  of  the  ""most 
familiar  and  beautiful  of  the* 
microscopic  fresh -water  organ- 
isms, we  find  the  individual 
cells,  each  one  still  closely 
resembling  a  Haematococcus, 
arranged  side  by  side  in  the 
gelatinous  wall  of  a  hollow 
sphere,  with  their  flagella  pro- 
jecting from  the  surface,  while 
daughter  colonies  are  frequently  seen  swimming  about  freely  in 
the  interior  of  the  sphere. 

In  all  these  colonies  the  gelatinous  matrix  or  ground  substance 
is  formed  as  a  secretion  by  the  cells  which  it  serves  to  hold 
together.  It  is  noteworthy  that  the  connection  between  the 
individual  cells  is  much  more  intimate  in  Volvox  than  it  is  in  the 
lower  types,  for  they  are  all  united  together  by  extensions  of 
their  protoplasmic  bodies  which  give  a  reticulate  appearance  to 
the  wall  of  the  sphere.  Volvox,  moreover,  attains  a  relatively 
large  size,  and  there  may  be  as  many  as  22,000  cells  in  a  single 
colony,  though  about  half  that  number  appears  to  be  more 
usual. 

We  meet  with  another  example  of  the  formation  of  hollow 
spherical  colonies  in  the  case  of  the  beautiful  Radiolarian 


FlG.  10. — Pandorina  morum,  X  400. 
(From  Vines'  "  Botany.") 

A,  free-swimming  colony ;  B,  conjugation 
of  two  gametes. 


RESULTS   OF   COLONY  FORMATION 


43 


Sphserozoum  (Fig.  12),  which  floats  about  in  the  surface  waters  of 
the  open  ocean  and  has  in  each  of  its  component  cells  a  supporting 
skeleton  of  branched  siliceous  spicules. 

The  co-operation  of  a  larger  or  smaller  number  of  cells  to  form 
a   colony  at   once  opens   up  new   possibilities   with   regard   to 


FIG.  11. — Volvox  aureus.     (From  "Weismann's  "Evolution  Theory,"  after 
Klein  and  Schenck.) 

A,  mature  colony  containing  daughter  colonies  (t)  and  ova  (o) ;  23,  group  of  32  developing 
spermatozoa  seen  end  on ;  C,  the  same  seen  sideways ;  X>,  mature  spermatozoa, 
x  824. 

, 

differentiation  and  division  of  labour.  If  a  sufficiently  good 
understanding,  so  to  speak,  can  be  established  between  the 
different  members  (or  zooids)  qf  the  colony  it  will  no  longer  be 
necessary  for  each  one  to  do  everything  for  itself.  At  the  expense 
of  becoming  mutually  dependent  upon  one  another  they  will  be 
able  to  specialize  in  different  directions,  some  identifying  them- 
selves with  one  necessary  duty  or  function  and  some  with 


44 


OUTLINES  OF  EVOLUTIONARY  BIOLOGY 


another.     As  a  result  of  this  specialization  the  various  functions 

may  be  much  more  efficiently  and  economically  performed,  but  a 

no  less  important  result  will  be  that  the  different  individuals  will 

i  no  longer  be  able  to  lead  independent  lives — if  separated  from 

one   another  they  will    perish  because   unable  to  perform    for 

'  themselves  individually  all  the  functions  which  are  necessary  for 

their  existence. 

In  colonies  of  Protista  we  meet  with  little  if  any  of  this 
differentiation  and  division  of  labour ;  from  the  physiological 
point  of  view  the  cell  units  remain  almost  if  not  quite  inde- 
pendent of  one  another,  and  that  is  why  they  are  still  classed 

amongst  the  unicellular  organ- 
isms. There  can  be  no  doubt, 
however,  that  it  was  this  habit 
of  colony-formation  that  led  to 
the  origin  of  true  multicellular 
animals  and  plants — Metazoa 
and  Metaphyta — from  unicel- 
lular ancestors.  The  compo- 
nent cells  of  a  colony  gradually 
became  integrated  to  form  an 
individual  of  a  higher  order, 
and  this  process  was  accom- 
panied by  that  differentiation 
and  division  of  labour  which 
in  course  of  time  led  to  the 
astonishing  complexity  of 
structure  which  characterizes 
the  higher  members  of  both  the  animal  and  vegetable  kingdoms, 
In  contrast  to  this  complexity  of  the  organism  as  a  whole  we 
shall  find  that  the  individual  cells  of  one  of  the  higher  animals 
or  plants  are  usually  much  simpler  in  structure  than  the  more 
highly  organized  Protista.  After  what  we  have  said  about 
differentiation  and  division  of  labour  the  reason  for  this  should 
be  sufficiently  obvious. 

Perhaps  no  more  convincing  demonstration  of  the  applicability 
of  the  cell  theory  to  the  higher  plants  and  animals  could  be 
given  than  that  which  is  afforded  by  the  study  of  development, 
for,  however  highly  organized  a  plant  or  an  animal  may  be  in  the 
adult  condition,  it  always  commences  its  individual  existence  as 
a  single  cell — the  fertilized  ovum  or  zygote— and  attains  the 


FIG.  12. — A  colony  of  Sphserozoum, 
cut  in  half.  (After  Haeckel,  in 
"  Challenger  "  Eeport.) 


SEGMENTATION   OF   THE    OVUM  45 


bdult  state  by  means  of  a  longer  or  shorter  series  of  cell-divisions, 
earlier  cell-divisions  constitute  what  is  termed  the  segmenta- 
jion  of  the  ovum,  and  result  in  the  formation  of  a  number  of 
laughter  cells,  which  exhibit  little  or  no  differentiation  amongst 
themselves  and  are  known  as  blastomeres.  Sooner  or  later, 
lowever,  differentiation  sets  in  and  leads  to  the  formation  of 
nore  or  less  highly  specialized  tissues  or  cell  aggregates.  * 

In  the  primitive,  fish-like  Amphioxus,1  for  example,  the  ovuna^ 
JFig.  13,  I)  is  a  spherical,  nucleated  cell  about  ^Jotn  incn  *n 
[diameter.  After  fertilization  by  a  male  gamete  or  spermatozoon 
it  divides  first  into  two  equal  and  similar  embryonic  cells  or 
blastomeres  (Fig.  13,  II)  by  a  vertical  cleavage.  Another  vertical 
cleavage,  at  right  angles  to  the  first,  divides  each  blastomere 
I  into  two  smaller  ones  (Fig.  13,  III).  This  is  followed  by  a  hori- 
zontal cleavage,  which  results  in  the  formation  of  eight  cells, 
iii  two  tiers  of  four  each,  the  four  upper  ones  being  slightly 
smaller  than  the  four  lower  (Fig.  13,  IV).  The  blastomeres  go  on 
dividing  and  presently  arrange  themselves  in  the  form  of  a 
hollow  sphere,  whose  wall  is  composed  of  a  single  layer  of  cells 
(Fig.  13,  VII).  The  embryo  has  now  reached  the  blastula  (or 
blastosphere)  stage  of  its  development,  a  stage  which  is  passed 
through  by  all  multicellular  animals  whose  life-history  follows 
a  typical  course  unmodified  by  secondary  features. 

If  we  allow  for  the  fact  that  the  cells  all  remain  together 
instead  of  separating  from  one  another  after  each  division,  it  is 
obvious  that  the  segmentation  of  the  fertilized  ovum  into  blasto- 
meres is  identical  with  the  process  of  multiplication  by  fission  in 
such  a  protozoon  as  Amoeba.  The  fact  that  each  blastomere  is 
equivalent  to  a  single  protozoon  and  multiplies  in  a  similar 
manner  has  been  experimentally  demonstrated  in  an  extremely 
interesting  way  by  Herbst,  whose  observations  were  made  upon 
the  development  of  the  common  sea  urchin  (Echinus).  He 
found  that,  if  the  eggs  are  allowed  to  develop  in  sea  water  from 
which  every  trace  of  calcium  has  been  removed,  the  blastomeres 
actually  do  separate  after  each  division  and  give  rise  to  808 
individual  cells,  which  swim  about  separately  like  so  many 
flagellate  Protozoa  instead  of  remaining  united  together  and 
co-operating  with  one  another  to  form  the  normal  blastula.  The 
blastula  stage  itself,  of  course,  corresponds  very  closely  in  general 
features  with  such  a  protozoon  colony  as  we  meet  with  in  the  case 

1  The  external  appearance  of  the  adult  Amphioxus  is  represented  in  Fig.  118. 


OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


FIG.   13. — Early  Development  of  Amphioxus.     (Adapted  from  Ziegler's 
Models,  after  Hatschek.) 

I,  the  fertilized  egg ;  II,  two  blastomeres  formed  by  the  first  cleavage  ;  III,  stage  with 
four  blastomeres;  IF,  stage  with  eight  blastomeres;  F,  stage  with  sixteen  blasto- 
meres ;  FJ,  stage  with  thirty-two  blastomeres,  cut  in  half  vertically  ;  FIT,  blastula  or 
blastosphere  stage,  cut  in  half ;  VIII,  early  stage  of  gastrulation,  cut  in  half ; 

\ 


BLASTULA  AND  GASTRULA         47 

>f  Volvox  or  Sphserozoum  (compare  Figs.  11  and  12),  a  point  to 
,vhich  we  shall  have  occasion  to  return  in  a  later  chapter. 

In  the  blastula  of  Amphioxus  the  cells  are  still  all  very  much 
ilike,  except  that  those  at  one  pole  of  the  sphere  are  somewhat 
arger  than  the  others.  Differentiation,  however,  now  sets  in  in 
i  very  marked  manner,  and  the  cells  thereby  become  divided  into 
wo  distinct  groups.  That  portion  of  the  wall  of  the  hollow 
>lastula  which  is  formed  by  the  larger  cells  becomes  pushed 
nwards  or  invaginated  (Fig.  13,  VIII),  much  as  a  tennis  ball 
may  be  pushed  in  by  the  pressure  of  the  thumb,  until  it  comes 
nto  contact  with  the  inner  surface  of  the  remainder  of  the  wall. 
In  this  way  the  original  cavity  (blastocoel)  is  obliterated  and 
he  embryo  takes  on  the  form  of  a  double  cup  (Fig.  13,  IX,  X). 
The  cavity  of  this  cup  is  an  entirely  new  formation.  It  is  the 
Drimitive  digestive  cavity  of  the  animal  and  is  known  as  the 
nteron  or  gastral  cavity.  Its  mouth  gradually  contracts  to  a 
narrow  aperture,  the  blastopore.  The  outer  layer  of  cells  forming 
he  wall  of  the  cup  is  termed  the  epiblast  and  the  inner  the 
hypoblast,  and  the  two  are  continuous  with  one  another  all  round 
he  blastopore.  The  stage  now  reached  is  spoken  of  as  the 
gastrulajtage.  nuu^v^ 

In  nearly  all  Metazoat/ the  blastula  stage  of  development  is 
ollowed  by  one  exhibiting  the  ftgapmtfal  features  nf  tV>A-ga.Rirn1a.) 
or  at  any  rate  some  indication  thereof.  The  primary  differentia- 
ion  of  the  component  cells  of  the  body  into  an  outer  epiblast 
which  becomes  the  ectoderm  of  the  adult)  and  an  inner 
lypoblast  (which  becomes  the  endoderm  of  the  adult),  the  one 
serving  for  protection  and  for  the  maintenance  of  all  the 
necessary  relations  with  the  external  environment,  and  the  other 
surrounding  a  gastral  cavity  and  concerned  with  the  digestion  of 
jhe  solid  food  which  the  animal  captures,  is  closely  correlated 
with  the  characteristic  animal  or  holozoic  method  of  nutrition. 

In  later  stages  of  development,  in  all  animals  higher  than  the 


IX,  young  gastrula  in  longitudinal  section  ;  X,  older  gastrula  in  longitudinal  section  £ 
XI,  XII,  XIII,  transverse  sections  of  older"  embryos,  showing  the  formation  of  the 
coelomic  pouches,  notochord  and  neural  tube ;  XI V,  longitudinal  section  of  embryo 
of  about  the  same  age  as  XII;  XV,  side  view  of  embryo  of  same  age  as  .XTFwith 
the  epiblast  stripped  off  from  one  side  to  show  the  mesoblastic  somites  formed  from 
the  coelomic  pouches. 

bp.  blastopore ;  blc.  blastocoel  or  segmentation  cavity;  c.p.  coelomic  pouch;  ent,  enteron  ; 
ep.  epiblast ;  hyp.  hypoblast ;  m.s.  mesoblastic  somites  ;  not.  notochord  ;  n.p.  neuro- 
pore  or  anterior  opening  of  neural  tube;  n.pl.  neural  plate;  n.t.  neural  tube  (central 
nervous  system,  formed  by  folding  of  neural  plate) ;  o.c.p.  openings  of  coelomic 
pouches  into  enteron. 


48          OUTLINES   OF   E VOLUTION AKY  BIOLOGY 

Coelenterata  (a  group  which  includes  such  forms  as  jelly-fish,  sea 
anemones  and  corals),  a  third  layer  of  cells  makes  its  appearance 
between  the  first  two  and  forms  the  mesoblast  (giving  rise  to  the 
mesoderm  of  the  adult).  In  Amphioxus  this  mesoblast  arises  as 
a  series  of  hollow,  pouch-like  outgrowths  of  the  hypoblast  (Fig.  13, 
XI,  XII,  c.p.),  and  the  cavities  which  these  pouches  contain  give 
rise  to  the  body  cavity  or  coelom,  while  their  walls  give  rise  to 
mesodermal  tissues. 

In  accordance  with  what  is  commonly  known  as  the  germ- 
layer  theory,  these  three  layers  of  cells,  epiblast,  mesoblast,  and 
hypoblast,  can  be  recognized  in  the  embryos  of  all  the  higher 
animals,  and  from  them  all  the  various  parts  of  the  adult  body 
are  derived.  ( The  epiblast  gives  rise  to  the  outer  skin  or 
epidermis,  the  nervous  system  and  the  essential  parts  of  the 
sense-organs ;  the  hypoblast  gives  rise  to  the  lining  epithelium  of 
the  alimentary  canal  and  its  various  outgrowths,  including  the 
digestive  glands,  while  from  the  mesoblast  arise  the  various 
connective  and  skeletal  tissues,  the  blood-vessels  and  the  essential 
>rgans  of  reproduction,  j  The  cells  of  which  each  tissue  is  com- 
posed acquire  a  characteristic  structure  of  their  own,  and  in 
this  way  the  histological  differentiation  of  the  body  is  gradually 
completed. 

With  the  early  stages  in  the  development  of  Amphioxus, 
described  above,  we  may  compare  the  corresponding  processes  in 
the  life-history  of  a  flowering  plant.  Here  the  ovum,  instead  of 
being  at  once  liberated  from  the  parent,  undergoes  all  the 
earlier  stages  of  its  development  within  the  so-called  ovule, 
which,  with  the  contained  embryo,  will  ultimately  be  set  free  as 
the  ripe  seed  (compare  Chapter  VIII.). 

As  a  definite  example  we  may  take  the  common  weed  known  as 
shepherd's  purse,  Capsella  bursa-pastoris  (Fig.  14).  The  ovum 
lies  in  a  cavity  in  the  ovule  termed  the  embryo-sac.  It  is  at  first 
a  single  nucleated  mass  of  protoplasm  without  any -  cell-wall. 
After  fertilization  it  divides  into  two  cells  separated  by  a  wall, 
and  the  process  is  repeated  by  a  series  of  divisions  parallel  to  the 
first  one  until  we  have  a  row  of  cells.  The  cell  at  one  end  of 
this  row,  known  as  the  basal  cell  (Fig.  14,  A,  B,  I.e.},  is  larger 
than  the  others  and  is  attached  to  the  wall  of  the  embryo-sac. 
At  the  opposite  end  is  a  rounded  cell,  called  the  embryonic  cell, 
from  which  the  body  of  the  young  plant  will  be  chiefly  formed. 
The  remainder  of  the  row,  including  the  basal  cell,  is  known  as 


EARLY  DEVELOPMENT   OF  CAPSELLA 


49 


the  suspensor,  and  serves  for  the  attachment  and  nutrition  of  the 
embryo,  which  at  first  develops  entirely  at  the  expense  of  the 
parent,  having  no  means  of  feeding  itself.  All  the  cell-divisions 


/ 


FIG.  14.— Four  stages  in  the  early  Development  of  a  Flowering  Plant, 

-"**™10      v     ^^^          /"TiVn-m    «nr»ff'c    f(  Sfnipfnrnl    Tlntanv." 


Capsella  bursa-pastoris,   X 
after  Hanstein.) 


(From  Scott's  "  Structural 


I.e.,  basal  cell  of  suspensor;  c£.,  cotyledons  growing  out ;  d,  dermatogen;  et  embryonic 
group  of  cells;  g.p.,  growing  point  of  stem;  h,  uppermost  cell  of  suspensor; 
pe,  periblem ;  pr,  pi,  cells  of  plerome ;  «lj  s^  cells  derived  from  ~h . 

so  far  have  taken  place  in  planes  parallel  to  one  another,  thus 
giving  rise  to  a  single  row  of  cells,  but  the  embryonic  cel^jnow 
divides  into  four  parts  by  the  formation  of  two  cell-walls  at  right 
angles  to  each  other  and  to  the  preceding  diyis^fls  (Fig.  14, 
A,  e),  .and  each  of  these  again  divides  into  tjvor»JifTh"e  formation 
B.  i  E 


SO         OUTLINES   OF  EVOLUTIONABY  BIOLOGY 

of  a  wall  parallel  to  the  earlier  divisions  (Fig.  14,  B,  e).  The 
embryo  now  consists  of  eight  cells  or  "  octants."  The  next  divi- 
sions are  parallel  to  its  surface  and  cut  off  a  superficial  covering 
of  cells,  known  as  the  dermatogen  (Fig.  14,  C,  d),  from  which 
the  whole  of  the  epidermis  of  the  adult  shoot  will  be  derived. 
The  mass  of  rapidly  dividing  cells  within  this  soon  becomes 
differentiated  into  two  parts,  the  plerome  (Fig.  14,  D,pr,pl),  lying 
in  the  axis  of  the  embryo,  and  the  periblem  (Fig.  14,  D,  pe),  lying 
between  the  plerome  and  the  dermatogen.  The  plerome  will 
give  rise  to  the  vascular  system  of  the  plant  and  the  tissues  asso- 
ciated therewith,  while  the  periblem  will  give  rise  to  the  cortical 
tissues  of  the  stem  and  root  and  the  mesophyll  or  middle  layer  of 
the  leaves.  The  periblem  of  the  root,  and  the  root-cap,  are  really 
formed  from  the  uppermost  cell  of  the  suspensor  (Fig.  14,  C,  h). 
By  further  cell-multiplication  and  differentiation  in  the  three 
primary  layers — dermatogen,  periblem  and  plerome — all  the 
various  tissues  of  the  adult  plant  are  produced. 

Thus  we  see  that  in  the  higher  plants  and  animals  alike  the 
development  of  the  individual  from  the  fertilized  egg  consists  in 
the  first  place  of  a  process  of  cell-division,  and  in  the  second  plape 
of  differentiation  between  the  cells  thus  produced,  accompanied 
uy  grouping  of  the  differentiated  cells  to  form  the  more  or  less 
sharply  defined  tissues  of  the  adult. 


CHAPTER  V 

The  cell  theory  as  illustrated  by  the  histological  structure  of  the  higher 
animals  and  plants — Limitations  of  the  cell  theory — The  cell  as  the 
physiological  unit. 

IN  all  the  higher  animals  and  plants  the  constituent  cells  of 
the  adult  body  are  grouped  in  more  or  less  well  defined  tissues, 
which  originate  from  the  fertilized  ovum  in  the  manner  indicated 
in  the  last  chapter,  and  the  cells  of  each  tissue  co-operate 
with  one  another  in  the  fulfilment  of  some  common  function. 
The  study  of  the  microscopic  structure  of  tissues  is  termed 
histology. 

As  examples  of  animal  tissues  we  may  take  blood,  epithelium, 
fat,  cartilage,  muscular  tissue  and  nervous  tissue,  as  met  with 
in  typical  vertebrates. 

Blood  is  exceptional  in  that  it  is  a  liquid  tissue,  a  condition 
which  is  of  course  necessary  in  order  that  it  may  circulate  through 
the  blood-vessels  and  perform  its  functions  as  -the  distributor 
throughout  the  body  of  food  material  and  oxygen,  arid  the  carrier 
of  carbon  dioxide  and  other  waste  products  from  all  the  various 
parts  of  the  body  to  the  special  organs  of  respiration  and  excre- 
tion. Floating  in  the  liqui^jsortion,  or  plasma,  are  found  two 
kinds  of  cells,  the' white  and- fed  blood-corpuscles. 

The  white  corpuscles,  or  leucocytes  (Fig.  15,  a.),  closely 
resemble  Amoeba3.  They  are  colourless,  nucleated  cells,  exhibiting 
amoeboid  movements,  and  they  have  the  remarkable  power  of 
creeping  through  the  thin  walls  of  the  blood-capillaries  into  the 
surrounding  tissues.  Like  the  Amoeba  they  feed,  in  part  at  any 
rate,  by  taking  in  and  digesting  the  bodies  of  other  minute 
organisms,  and  they  grow  and  multiply  by  simple  fission.  They 
exhibit  a  much  greater  degree  of  independence  than  most  of  the 
cells  of  the  body  and  can  even  live  outside  the  body  for  a  time  if 
kept  in  suitable  culture  media  and  at  the  proper  temperature. 
Their  most  important  function  appears  to  be  to  defend  the  body 
from  the  attacks  of  bacteria  and  other  harmful  micro-organisms. 

E  2 


i 


52          OUTLINES   OF   E  VOLUTION  AKY  BIOLOGY 

If  these  gain  entrance  into  the  tissues  at  any  weak  spot  they  are 
set  upon  by  the  leucocytes  and  literally  devoured.  This  process 
is  known  as  phagocytosis,  and  in  this  capacity  the  leucocytes  are 
often  spoken  of  as  phagocytes.  It  is  obvious  that  the  health  of 
the  body  must  depend  largely  upon  the  activity  of  the  phagocytes 
and  their  efficiency  in  dealing  with  disease-producing  "  germs." 

The  white  blood-corpuscles,  then,  differ  in  no  essential  parti- 
cular as  regards  their  structure  and  mode  of  life  from  so  many 
Protozoa.  It  is  true  they  cannot  live  permanently  outside  the 


® 

a.  1& 

FIG.  15. — Blood  Corpuscles  of  the  Frog,  X  326.     (From  a  photograph.) 

a.,  white  corpuscle  or  leucocyte;  b.,  red  corpuscles  or  haematids.  The  nuclei  appear 
light-coloured  in  the  photograph  owing  to  their  having  been  stained  blue  in  the 
preparation. 

body,  but  that  is  also  the  case  with  many  parasitic  Protozoa 
which  live  in  the  blood  of  other  animals.  That  they  are  not 
independent  organisms,  but  form  an  integral  part  of  the  body 
in  which  they  occur,  is,  however,  obvious  from  the  fact  that 
they  have  a  common  origin  with  all  the  other  tissues  from  the 
developing  ovum. 

The  red  corpuscles,  or  haematids,  are  very  different  bodies. 
They  float  passively  in  the  blood-stream  and  serve  as  the  carriers  of 
oxygen  gas  from  the  respiratory  organs  to  the  various  tissues. 
Unlike  the  leucocytes  they  have  definite  and  constant  outlines, 
though,  owing  to  the  flexible  nature  of  the  thin  cell-membrane  by 
which  they  are  enclosed,  they  may  undergo  temporary  distortion. 


HISTOLOGY   OF  HIGHEK  ANIMALS 


53 


Individually  of  a  pale  yellow  colour,  and  so  small  as  to  be  quite 
invisible  to  the  naked  eye,  they  occur  in  such  vast  numbers  as  to 
give  the  blood  its  characteristic  scarlet  or  purple  colour.  It  is 
estimated  that  in  a  cubic  millimetre  of  human  blood  there  are 
about  five  millions  of  these  red  corpuscles. 

In  the  frog  the  haematids  are  flattened  oval  cells  about  0'02  mm. 
in  longer  diameter,  with  a  centrally  placed  nucleus  (Fig.  15,  b.). 
In  man  they  are  a  good  deal  smaller,  and  circular  in  out- 
line, like  biscuits,  and,  as  in  all  the  Mammalia,  the  nucleus 


FIG.  16.— Epithelium  from  the  Mesentery  of  a  Frog,    X   280.     (From  a 

photograph.) 

The  underlying  tissues  are  seen  indistinctly  through  the  transparent  epithelial  cells, 
whose  outlines  only  are  visible. 

lias  entirely  disappeared.  They  owe  their  red  colour,  and 
their  power  to  act  as  carriers  of  oxygen,  to  the  presence  in  them 
of  a  peculiar  pigment  known  as  haemoglobin,  with  which  the 
oxygen  appears  to  enter  into  a  state  of  loose  chemical  combination 
from  which  it  is  easily  liberated  again  when  required  by  the  tissues. 
They  may  indeed  be  regarded  as  mere  bags  of  haemoglobin,  formed 
from  highly  specialized  cells  which  have  lost  all  power  of  indepen- 
dent existence.  They  cannot  even  multiply  by  division,  but,  as 
the  old  ones  are  worn  out,  they  are  replaced  by  the  formation 
of  new  ones  from  less  specialized  cells  in  various  parts  of  the 
body. 


54 


OUTLINES  OF  EVOLUTIONARY  BIOLOGY 


The  term  epithelium  is  applied  to  any  layer  of  cells  covering, 
a  free  surface.  An  epithelium  is  therefore  primarily  a  protective 
layer,  but  it  frequently  becomes  modified  for  other  purposes.  It 
may,  for  example,  become  glandular,  certain  of  its  cells  taking  on 
the  function  of  secretion,  or  it  may  become  sensory,  with  cells 
specially  adapted  for  the  reception  of  stimuli.  It  may  consist  of 
a  single  layer  of  cells  or  of  several  layers  one  above  the  other. 

A  good  example  of  the  single-layered  type  is  found  in  the 
peritoneal  epithelium  which  covers  the  surface  of  the  mesentery 
or  membrane  supporting  the  intestines  in  the  coelom  or  body 
cavity.  Fig.  16  represents  a  portion  of  such  an  epithelium  in 

which  the  cell -outlines  have 

^^^^^^fj^^^^Si  been  rendered  very  distinct  ( 

by  staining  with  silver 
nitrate;  the  nuclei,  how- 
ever, are  not  shown  by  this 
method.  Each  cell  has  the 
form  of  a  thin,  flat,  poly- 
gonal plate,  and  they  all 
fit  accurately  together  at 
their  edges.  With  this 
Figure  should  be  compared 
Fig.  28,  A,  which  represents 
a  single-layered  epithelium 
prepared  in  such  a  way  as 
to  show  both  nuclei  and  cell- 
outlines. 

If  we  gently  scrape  the  inside  of  the  cheek  with  some  clean, 
blunt  instrument,  and  examine  the  milky-looking  product  unde, 
the   microscope,   we  shall  find   that    it  contains   a   number  o 
flattened,  scale-like  bodies  (Fig.   17),  either   entirely  separate* 
from  one  another  or  still  more  or  less  connected  together  by  thei 
edges,  and  probably  to  some  extent  overlapping.     These  also  ar< 
epithelial  cells,  which  have  formed  part  of  the  special  epithelium 
known  as  the  epidermis,  which  is  derived  from  the  epiblast  or 
external  cell -layer  of  the  embryo  and  covers  the  outer  surface  of 
the  body.     If  we  examine  our  preparation  more  carefully  we  shall 
find  that  the  cells  have  an  irregularly  rounded  contour  and  that 
they  measure  about  0'08  mm.  in  diameter.     There  is  a  more  or 
less  centrally  placed  nucleus  (which  appears  dark  in  the  figure 
owing  to  the  manner  in  which  it  has  been  stained)  and  the 


FIG.  17.— Five  isolated  Epithelial  Cells 
from  the  inner  Surf  ace  of  the  human 
Cheek,  X  420.  (From a  photograph.) 
The  nuclei  are  stained  darkly. 


HISTOLOGY  OF   HIGHER  ANIMALS 


55 


iytoplasm  is  granular.     They  are,  however,  dead,  or  at  any  rate 
moribund  cells.     Owing  to  the  constant  friction  to  which  the 


---  s.m.j 


er.< 


FIG.  18.— Vertical  Section  of  Stratified  Epithelium  from  the  Mouth  of  a 
foetal  Cat,  X  280.     (From  a  photograph.) 

der.,  dermis;  epd.,  epidermi%<;  s.m.,  Stratum  Malpighii,  or  layer  of  actively  dividing  cells. 

surface  of  the  body  is  exposed  such  cells  are  always  being  rubbed 
off,  and  it  is  these  which,  by  accumulating  in  places  where  the 
friction  is  less  severe,  form  the  so-called  scurf  of  the  hair. 


FIG.  19. — Section  of  Adipose  Tissue  from  which  the  Fat  has  been  dissolved 
out,  leaving  the  thin-walled  Cells  empty  and  shrivelled,  X  175.  (From 
a  photograph.) 

As  they  are  worn  away  their  places  are  taken  by  other  cells 
which  arise  from  a  deeply  situated  layer,  at  the  lower  limit  of  the 


56          OUTLINES   OF   EVOLUTIONAKY  BIOLOGY 

epidermis,  in  which  cell-division  goes  on  actively  throughout  life. 
In  this  way  a  many-layered  or  stratified  epithelium  is  formed,  as 
shown  in  Fig.  18,  which  represents  a  small  portion  of  a  thin 
vertical  section  through  the  epidermis  (epd.)  in  the  mouth  of  a 
foetal  cat.  At  the  lower  limit  of  the  epithelium,  resting  imme- 
diately upon  the  connective  tissue  of  the  dermis  (der.),  is  seen 
the  layer  of  actively  dividing  cells  (s.m.).  The  cells  cut  off  from 
this  layer  are  gradually  pushed  outwards,  becoming  flattened  and 
scale-like  as  they  approach  the  surface. 

Fat,  or  adipose  tissue,  consists  of  an  aggregation  of  more  01 


FIG.  20. — Section  of  Cartilage,  showing  the  Cartilage  Cells  (c.c.)  imbedded  in 
the  transparent  intercellular  Matrix  or  Ground  Substance  (m.),  X  390. 
(From  a  photograph.) 

less  globular  cells,  swollen  out  by  the  accumulation  within  them 
of  drops  of  oil.  If  the  oil  is  dissolved  out  by  suitable  reagents 
the  empty  cells  are  left  with  their  cell-walls  or  membranes  in  a 
somewhat  shrivelled  condition,  as  shown  in  Fig.  19,  and  the 
tissue  now  bears  a  curious  resemblance,  when  seen  in  section,  to 
vegetable  parenchyma,  such  as  is  seen  in  sections  of  pith 
(compare  Fig.  7). 

Cartilage,  or  gristle,  is  one  of  the  skeletal  tissues,  serving  for 
the  support  of  the  body  and  the  protection  of  special  organs. 
In  some  of  the  lower  vertebrates,  such  as  the  dog-fish,  it  forms 
practically  the  whole  of  the  internal  skeleton,  but  in  higher  forms 


HISTOLOGY   OF   HIGHER  ANIMALS 


57 


it  is  to  a  greater  or  less  extent  supplemented  or  even  replaced  by 
bone.  It  consists  mainly  of  a  tough,  translucent  matrix,  or 
intercellular  substance,  which  is  formed  as  a  secretion  by  the 
cartilage  cells  and  in  which  the  latter  are  imbedded  at  wide 
intervals  (Fig.  20).  In  this  respect  it  differs  greatly  from  the 
epidermis,  in  which  the  cells  lie  close  together  and  little  or  no 
intercellular  substance  is  developed.  The  cartilage  grows  by 
repeated  division  of  the  cells  which  it  contains  and  the  secre- 
tion of  additional  intercellular  matrix  between 
them.  The  frequent  arrangement  of  the 
nucleated  cells  in  pairs,  as  shown  in  the 
illustration,  is  an  indication  of  recent  cell- 
division.  Bone  is  a  more  complex  tissue  than 
cartilage  and  is  further  strengthened  and 
hardened  by  the  deposition  of  calcareous 
salts,  chiefly  phosphate  of  lime,  in  the  matrix. 

Muscular  tissue  is  specialized  in  a  totally 
different  direction  from  any  of  the  foregoing. 
Its  function  is  to  contract,  and  by  so  doing 
to  bring  about  the  various  movements  of 
which  the  higher  animals  are  capable.  There 
are  two  very  distinct  kinds  of  muscular  tissue, 
the  one  comparatively  simple  and  the  other 
much  more  complex  in  structure.  The  former, 
which  is  known  as  unstriped  muscle  (Fig.  21), 
consists  of  greatly  elongated  cells,  the  muscle- 
fibres,  associated  in  sheets  or  bundles.  Each 
has  a  centrally  placed  nucleus  and  its  cellular 
nature  is  at  once  obvious.  The  wall  of  the 
alimentary  canal,  outside  its  lining  epithelium, 
is  composed  chiefly  of  muscle-fibres  of  this 
kind.  Their  rhythmical  and  co-ordinated  contraction  causes 
the  characteristic  peristaltic  movement  whereby  the  onward 
passage  of  the  food  is  secured.  These  and  similar  movements 
effected  in  other  organs  by  the  action  of  unstriped  muscular 
tissue  take  place  quite  independently  of  the  will — whence  the 
term  "  involuntary  "  is  often  applied  to  this  type  of  muscle. 

Striped  or  striated  muscular  tissue  is  usually  under  the  control 
of  the  will,  and  is  hence  often  spoken  of  as  "  voluntary,"  but  it  is 
found  in  the  higher  animals  wherever  very  sharp,  precise  move- 
ments are  required,  as  for  example  in  the  walls  of  the  heart,  the 


FiG.21.— Uustriped 
Muscle  Fibres 
from  the  Wall  of 
the  Eabbit's  In- 
testine, X  300. 

nu.,  nuclei. 


58 


OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


contraction  of  which  serves  to  pump  the  blood  through  the  blood- 
vessels. It  is  more  especially  associated,  however,  with  the 
movements  of  the  limbs,  the  bones  of  which  form  a  system  of 
levers  operated  by  the  muscles  which  are  attached  to  them.  It 
differs  greatly  in  minute  structure  from  unstriped  muscle,  though 
consisting  essentially  of  greatly  elongated,  nucleated  fibres 
endowed  with  remarkable  powers  of  contraction.  These  fibres, 
and  the  fibrillae  into  which  they  are  subdivided,  are  characterized 
by  a  transverse  striation  of  alternate  light  and  dark  bands. 
Their  structure  is  very  complex  and  in  the  fully  developed 
muscle  it  is  difficult  if  not  impossible  to  recognize  the  limits 


FIG.  22. — Striped  Muscle-Fibres  (m.)  from  the  Tail  of  a  larval  Axolotl, 
showing  their  nuclei  (n.),  X  560.     (From  a  photograph.) 

between  the  constituent  cells.  Fig.  22  represents  a  number  of 
striated  muscle-fibres  from  the  tail  of  a  larval  axolotl,  in  which 
each  fibre  is  seen  to  be  provided  with  several  distinct  nuclei. 

The  nervous  system,  as  we  have  already  pointed  out,  serves  to 
place  the  different  parts  of  the  body  in  communication  with  one 
another  and  exercises  a  controlling  and  co-ordinating  influence 
over  the  whole,  while  through  the  mediation  of  the  special  organs 
of  sense  it  keeps  the  organism  in  close  touch  with  its  environ- 
ment. The  tissue  of  which  it  is  composed  (Fig.  23)  consists 
\)f  nerve-cells  and  nerve-fibres,  but  the  fibres  are  merely  out- 
growths of  the  cells.  A  cell  and  fibre  together  form  a  neuron 
— a  single  unit  of  the  nervous  system.  The  nerve-cells,  or 
rather  their  bodies,  occur  chiefly  in  the  brain  and  spinal  cord, 
which  constitute  the  central  nervous  system,  but  also  in  small 


HISTOLOGY  OF   HIGHEE  ANIMALS  59 

local  aggregations,  or  ganglia,  in  various  parts  of  the  body. 
The  nerve-fibres  extend  outwards  from  the  central  nervous 
system  in  long,  slender  bundles,  the  nerves,  which  are  distributed 
to  the  various  organs. 

The  body  of  a  nerve-cell  contains  the  nucleus  and  is  usually 
much  branched  into  slender  processes  or  dendrons  (Fig.  24), 
which  are  quite  distinct  from  the  nerve-fibre  and  are  supposed 
to  afford  the  means  of  transmitting  impulses  between  one  nerve- 
cell  and  another,  with  the  dendrons  of  which  they  interlace. 


FIG.  23. — Nervous  Tissue,  as  seen  in  a  thin  Section  of  the  Brain  (Medulla 
oblongata)  of  the  Monk  Fish.  Two  large  nucleated  Nerve  Cells  are 
shown  imbedded  in  a  Mass  of  smaller  Cells  and  Fibres,  X  168.  (From  a 
photograph.) 

Like  other  higher  specialized  tissue-cells  of  the  animal  body  the 
neurons  have  lost  the  power  of  multiplication  by  division.  More- 
over there  appears  to  be  no  provision,  in  some  adult  vertebrates, 
for  their  renewal  when  worn  out  or  injured.  A  certain  number 
are  formed  in  the  course  of  the  development  of  the  embryo  and 
these  have  to  serve  the  animal  for  the  whole  of  its  life. 

All  the  different  kinds  of  cells  met  with  in  the  body,  a  few  of 
which  have  been  thus  briefly  described,  are  derived  from  the 
apparently  simple  unicellular  ovum  by  repeated  subdivision  and 
gradual  differentiation.  The  functions  which  in  an  Amceba  are 
all  performed  by  a  single  protoplasmic  unit  are  in  .one  of  the 


60 


OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


higher  animals  distributed  amongst  thousands  of  millions  of 
such  units,  arranged,  so  to  speak,  in  regiments  and  armies,  each 
group  with  its  own  duties  to  perform  and  all  co-operating  for  the 
common  good  under  the  supervision  and  control  of  the  central 
nervous  system.  The  marvellous  perfection  of  the  whole 
machinery  is  the  result  of  that  differentiation  and  division  of 
labour  which  was  first  rendered  possible  by  the  union  and  co- 
operation of  the  individuals 
of  a  protozoon  family  to  form 
a  multicellular  body. 
/"Turning  now  to  the  higher 
plants,  we  shall  find  that,  in 
accordance  with  their  much 
lower  degree  of  functional 
activity,  their  organization 
is  far  less  elaborate  than  in 
the  higher  animals.  In  cor- 
relation with  their  stationary 
habit  all  those  organs  and 
tissues  which  are  specially 
concerned  with  locomotion 
are  absent,  and  in  further 
correlation  with  this  character 
there  are  no  nervous  system 
and  no  special  organs  of  sense. 
Of  the  functions  concerned 
with  the  life  of  the  individual 
— that  is,  other  than  repro- 
ductive functions  —  that  of 
nutrition  is  alone  highly 


fftrctn- 
forts,ati 


FIG.  24. — Diagram  of  a  Neuron, 
(From  Hertwig.) 

Nervenfortsatz,  a  nerve-fibre  coming  off 
from  the  body  of  the  cell. 


developed.  The  entire  plant 
is  little  more  than  a  piece  of 
apparatus  for  extracting  carbon,  water  and  mineral  salts  from  the 
air  and  soil,  and  converting  these,  with  the  aid  of  the  sun's  rays, 
into  organic  substances.  Although  the  higher  plants  often  attain 
a  much  larger  size  than  any  animals,  this  does  not  indicate  a 
higher  degree  of  organization,  for  it  is  brought  about  simply  by 
the  repetition  of  similar  parts — such  as  roots,  branches  and 
leaves — and  the  accumulation  of  dead  cell-walls  in  the  form  of 
wood  and  bark.  As  we  have  already  seen,  a  green  plant,  instead 
of  spending  the  energy  which  it  derives  from  the  sun  on  its  own 


HISTOLOGY  OF   HIGHEE  PLANTS 


61 


activities,  stores  most  of  it  up  in  the  complex  chemical  compounds 
which  it  manufactures. 

Nevertheless,  though  the  degree  of  histological  differentiation 
is  not  nearly  so  high  as  it  is  in  the  higher  animals,  we  find  in 
the  higher  plants  also  a  considerable  variety  of  cells  and  tissues, 
derived,  as  we  have  already  pointed  out,  from  the  dermatogen, 
periblem  and  plerome  of  the  embryo. 

We  may  illustrate  this  point  by  a  study  of  some  of  the  cells 
and  tissues  which  occur  in  the  well-known  spiderwort  of  our 
gardens,  Tradescantia  virginica.  If  we  examine  the  flowers  of 
this  plant  we  shall  find  that  the  stamens  are  covered  with  long 


FIG.  25. — Structure  of  a  Hair  from  a  Stamen  of  Tradescantia  virginica. 

A.  End  of  a  hair  as  seen  under  a  low  power  of  the  microscope.    The  hair  is  made  up  of 

a  single  row  of  cells. 

B.  A  single  cell  more  highly  magnified. 

c.iv.  cell-wall;  nu.  nucleus;  p.u.  primordial  utricle;  vac.  vacuole  filled  with  coloured 
or  colourless  cell-sap. 

slender  hairs.  It  will  be  convenient  to  make  these  hairs  the 
starting  point  of  our  inquiry.  If  we  study  them  first  under  a  low 
magnifying  power  we  shall  see  that  each  hair  (Fig.  25,  A)  is 
made  up  of  a  single  row  of  cells,  arranged  like  the  beads  in  a 
necklace ;  most  of  the  cells  are  elongated,  but  towards  the  apex 
of  the  hair  they  become  short  and  spherical. 

If  we  now  concentrate  our  attention  on  one  of  the  larger  cells 
and  study  it  carefully  under  a  moderately  high  power  of  the 
microscope,  we  shall  find  that  it  exhibits  the  appearance  shown 
in  Fig.  25,  B.  It  measures  about  0'27  mm.  in  length  by 
0*08  mm.  in  breadth  and  consists  of  a  thin-walled  bag  filled 
with  living  protoplasm.  The  wall  (c.w.)  is  transparent  and 
colourless  and  is  composed,  as  in  Hsematococcus,  of  cellulose. 


62    OUTLINES  OF  EVOLUTIONABY  BIOLOGY 

The  granular,  colourless  protoplasm  does  not  fill  the  interior 
of  the  cell  in  a  uniform  manner,  but  is  arranged  partly  as  a 
thin  lining  to  the  cell-wall,  known  as^k^primordial  utricle 

wmc^^Wfc 


(p.u.),  and  partly  in  irregular  strings  wmcWfcnch  and  anas- 
tomose and  stretch  across  the  cavity  of  the  cell  in^MBuis 
directions.  These  strings  of  protoplasm  tend  to  converge  towWds 
an  irregular  mass  in  which  the  nucleus  is  situated.  The  nucleus 
itself  (nu.)  is  a  nearly  spherical  body  of  denser  protoplasm,  about 
0'024  mm.  in  diameter.  The  extensive  space  which  lies  i 
the  primordial  utricle  and  between  the  strands  of  protoplasm 
is  filled  with  a  more  fluid  liquid  known  as  the  cell-sap.  It  is 
to  this  cell-sap  that  the  flowers  of  Tradescantia  owe  their 
colour;  if  the  flowers  are  blue  the  cell-sap  will  be  found  to 
be  blue  and  if  they  are  white  it  is  because  the  cell-sap  is 
colourless. 

The  most  striking  feature  of  the  cell  which  we  are  examining 
still  remains  to  be  noticed.  The  protoplasm  is  in  constant  move- 
ment. This  is  at  once  evident  from  the  characteristic  streaming 
of  the  small  granules  which  it  contains.  Both  in  the  primordial 
utricle  and  in  the  network  of  threads  a  constant  circulation  is 
kept  up,  though  not  in  a  very  definite  manner,  as  the  threads 
themselves  are  constantly  undergoing  slow  changes  in  their 
arrangement.  The  arrows  in  Fig.  25,  B  indicate  approximately 
the  course  taken  by  the  streaming  protoplasm  at  the  time  when 
the  drawing  was  made, 

The  streaming  of  the  protoplasm  appears  at  first  sight  to  be 
an  essentially  vital  phenomenon,  but  it  is  probably  merely  the 
mechanical  result  of  chemical  and  physical  processes  going  on 
in  the  cell,  such  as  the  diffusion  of  various  substances  in  solution 
from(one  cell  to  another,  which  must  take  place  in  the  process  of 
nutrition.  If  the  cell  is  killed  by  the  addition  of  alcohol  the 
physical  and  chemical  conditions  are  at  once  altered  and  the 
movement  ceases  ;  the  protoplasm  is  coagulated  and,  if  we  are 
dealing  with  a  cell  containing  coloured  cell-sap,  the  nucleus 
absorbs  the  colouring  matter  with  great  avidity  and  becomes 
deeply  stained,  while  the  cytoplasm  stains  only  very  slightly  or 
not  at  all.  We  are  thus  able  to  make  the  cell  stain  itself 
differentially,  without  the  aid  of  any  extraneous  colouring  matter, 
the  cell-sap  acting  as  what  is  termed  a  nuclear  stain. 

It  will  be  necessary  to  restrict  our  further  observations  on 
Tradescantia  to  the  structure  of  the  leaf  (Fig.  26).  The  leaves 


HISTOLOGY  OF   HIGHER   PLANTS 


63 


of  this  plant  are  long  and  narrow,  somewhat  resembling  those  of 
grasses.     There  is  a  pronounced  midrib  and  the  so-called  veins 


FIG.  26. — Histology  of  the  Leaf  of  Tradescantia  virginica. 

A.  Part  of  a  transverse  section  of  the  leaf. 

B.  Piece  of  the  epidermis  stripped  from  the  lower  surface  of  the  leaf. 

C.  Chlorophyll  cells  from  the  mesophyll,  as  seen  in  a  longitudinal  section  of  the  leaf. 

D.  Portions  of  four  vessels  from  a  vascular  bundle,  with  spiral  and  annular  markings,  as 

seen  in  a  longitudinal  section  of  the  bundle. 

(All  more  or  less  highly  magnified.) 
a.c.  air-cavity;  c.c.  chlorophyll  cell;  cp.  chloroplastids ;  ep.c.  epidermic  cell;  epd.  epi- 
dermis; g.c.  guard-cell ;  mes.  mesophyll ;  nu.  nucleus ;  par.  colourless  parenchyma  ; 
sh.  sheath  of  vascular  bundle,  composed  of  thin- walled  cells  containing  starch  grains ; 
sJc.  thick-walled  skeletal  tissue;  st.  stoma;  v.b.  vascular  bundle;  ves.  vessels. 

run  parallel  with  one  another  from  base  to  apex,  as  in  all  typical^ 

Monocotyledons. 

i    If   we  cut   a   thin    transverse   section   of   a  living   leaf   and 

examine  it  under  the  microscope  in  a  drop  of  water  (Fig.  26,  A)  we 


64          OUTLINES   OF  EVOLUTIONARY   BIOLOGY 

shall  see  at  once  that  it  is  made  up  of  three  principal  tissue- 
systems,  the  epidermis,  the  mesophyll  and  the  vascular  bundles. 
The  epidermis  (epd.)  covers  the  upper  and  lower  surfaces  in  the 
form  of  two  single  layers  of  cells,  the  mesophyll  (mes.)  occupies  the 
space  between  these  two  layers,  and  the  vascular  bundles  (v.b.) — 
corresponding  to  the  veins  of  the  leaf — are  imbedded  in  the 
mesophyll  at  fairly  regular  intervals  (only  one  is  shewn  in  the 
figure).  We  must  examine  each  of  these  tissue-systems  sepa- 
rately, and  in  order  to  gain  a  correct  idea  of  the  form  and 
arrangement  of  the  cells  of  which  they  are  composed  it  will  be 
necessary  to  study  them  from  various  points  of  view. 

The  epidermis  may  be  stripped  off  bodily  from  the  surface  of 
the  leaf  and  then  exhibits  under  the  microscope  the  appearance 
shown  in  Fig.  26,  B.  It  is  composed  of  elongated  cells,  placed 
side  by  side  in  a  single  layer.  The  amount  of  protoplasm  which 
these  cells  contain  is  very  small,  but  the  nucleus  (nu.)  is 
frequently  conspicuous.  Their  external  walls  are  specially 
thickened  in  relation  to  their  protective  function,  a  feature 
which  can  only  be  seen  in  sections  (Fig.  26,  A).  At  frequent 
intervals  little  slit-like  openings  occur  in  the  epidermis.  These 
are  the  stomata  (st.)  which  lead  into  air-spaces  in  the  mesophyll. 
Each  stoma  is  bounded  by  a  pair  of  specially  modified  epidermic 
cells  known  as  the  guard-cells  (g.c.) — the  only  epidermic  cells 
containing  chlorophyll — which  have  the  power  of  opening  and 
closing  the  stoma  like  a  pair  of  lips  and  thus  regulating  the 
amount  of  aqueous  vapour  which  passes  through  the  stomata  in 
the  process  of  transpiration.  On  the  outer  side  of  each  guard- 
cell  lies  another  epidermic  cell  of  much  smaller  size  than  the 
ordinary  kind,  and  these,  together  with  the  guard-cells,  form  a 
kind  of  roof  (or  floor)  to  the  air-cavity  (Fig.  26,  A,  a.c.). 

The  mesophyll  contains  the  chlorophyll-bearing  cells,  by 
which  the  assimilation  of  carbon  dioxide  is  effected  and  which 
are  at  once  recognized  by  their  green  colour.  They  appear  more 
or  less  round  or  oval  in-transverse  sections  (Fig.  26,  A,  c.c.),  but 
are  really  considerably  elongated,  parallel  to  the  length  of  the 
leaf,  as  shown  in  Fig.  26,  C.  They  come  in  contact  with  one 
another  by  numerous  short  protuberances,  between  which  lie  the 
spaces  in  which  the  air,  containing  carbon  dioxide  and  aqueous 
vapour,  circulates.  Each  contains  a  nucleus  (nu.)  and  numerous 
small,  biscuit-shaped  green  bodies,  the  chloroplastids  or  chloro- 
phyll corpuscles  (cp.),  imbedded  in  the  cytoplasm. 


HISTOLOGY  OF   HIGHER  PLANTS  65 

In  certain  places  the  green  mesophyll  cells  are  interrupted  by 
groups  of  cells  containing  no  chlorophyll.  At  intervals  beneath 
the  upper  epidermis  we  see  masses  of  large,  thin-walled,  colourless 
cells  forming  a  parenchyma  or  ground- tissue  (par.),  while  beneath 
the  vascular  bundles  we  find  bands  of  very  thick-walled  cells  (sk.) 
which  play  an  important  part  in  the  mechanical  support  and 
strengthening  of  the  leaf. 

The  vascular  bundles  are  surrounded  each  by  a  sheath  of  thin- 
walled  cells  (sh.)  containing  numerous  small  starch  grains. 
Within  this  lie  the  bast  or  phloem  and  the  wood  or  xylem,  com- 
posed of  the  elongated  tubular  elements  through  which  the  sap 
circulates.  The  raw  sap  consists  of  water  with  mineral  salts 
in  solution,  and  ascends  through  the  xylem,  while  elaborated 
sap,  containing  the  proteids  which  have  been  manufactured  in 
the  leaf  under  the  influence  of  sunlight,  descends  through  the 
phloem.  For  our  present  purposes  we  may  confine  our  attention 
to  certain  of  the  xylem  elements,  known  as  the  spiral  and 
annular  vessels.  These  consist  really  of  dead  cell-walls,  forming 
long  narrow  tubes  each  composed  originally  of  a  row  of  cylindrical 
cells  placed  end  to  end.  In  the  course  of  their  development  the 
transverse  dividing  walls  between  these  cells  are  absorbed,  the 
protoplasm  disappears,  and  nothing  remains  but  a  long  hollow 
tube  whose  walls  are  strengthened  by  spiral  or  annular  thicken- 
ings. Portions  of  these  vessels  are  represented  separately  in 
Fig.  26,  D ;  in  A  they  are  seen  only  in  transverse  section  (ves.). 

Such  are  the  principal  kinds  of  tissue  met  with  in  a  typical 
flowering  plant,  and  such  is  the  way  in  which  it  carries  out  the 
principles  of  co-operation,  differentiation  and  division  of  labour 
amongst  its  constituent  cells. 

To  most  people  it  will  probably  appear  that  the  fundamental 
truth  and  general  applicability  of  the  cell  theory  are  sufficiently 
firmly  established  by  considerations  such  as  those  with  which  we 
have  been  dealing.  It  has,  however,  certain  undoubted  limita- 
tions, and  upon  these  limitations  some  biologists  are  inclined  to 
lay  a  good  deal  of  stress.  ^ 

Thus,  from  the  point  of  view  of  the  cell  theory,  we  regard  the 
cell  as  the  organic  unit ;  there  are,  however,  units  of  a  lower 
order,  of  which  the  cell  itself  is  composed.  The  chloroplastids  of 
one  of  the  higher  plants,  for  example,  exhibit  a  good  deal  of 
individuality,  being  capable  of  independent  growth  and  multi- 
plication, while,  as  we  shall  see  later  on,  it  is  necessary  for 

B.  F 


66 


OUTLINES   OF  EVOLUTIONAKY  BIOLOGY 


theoretical  reasons  to  postulate  the  existence  of  some  such  bodies 
as  Professor  Weismann's  biophors  as  the  primary  units  of  which 
living  protoplasm  is  built  up. 

Then,  again,  although  it  may  be  questioned  whether  any 
absolutely  unnucleated  organisms,  such  as  Haeckel's  Monera, 
really  exist,  there  can  be  no  doubt  that  the  most  simply  organized 
living  things  known  to  us,  the  Bacteria,  which  probably  stand  a 
long  way  below  the  point  where  the  animal  and  vegetable  king- 
doms part  company,  and  which  are  the  most  abundant  of  all 
living  organisms,  do  not  show  that  sharp  differentiation  into 
cell  body  and  nucleus  which  is  so  characteristic  of  typical  cells, 


FIG.  27. — Bacillus  saccobranchi.  Bacteria  from  the  Blood  of  a  Fish  (Sacco- 
branchus]  stained  so  as  to  show  the  distribution  of  the  chromatin 
(nuclear)  material,  which  is  represented  in  black,  and  which  may  be 
arranged  in  small  scattered  granules  throughout  the  cell,  or  in  an  irregular 
network,  or  in  an  irregular,  more  or  less  twisted  rod,  X  2000.  (After 
Dobell  in  the  "  Quarterly  Journal  of  Microscopical  Science.") 

the  nucj^ar  constituents  being  more  or  less  scattered  throughout 
the  cytoplasm  (Fig.  27). 

A  difficulty  of  another  kind  is  met  with  in  the  fact  that  in  a 
good  many  cases  the  division  of  the  nucleus  is  not  followed — at 
any  rate  not  immediately — by  corresponding  division  of  the 
cytoplasm.  Some  Amoebae  constantly  have  two  nuclei,  and  we  some- 
times get  relatively  large  masses  of  protoplasm  containing  many 
nuclei  formed  by  repeated  division.  These  are  termed  syncyjia. 
We  meet,  in  fact,  with  all  degrees  of  separation  of  the  cytoplasm 
into  distinct  cells,  and  a  great  many  of  the  cells  even  of  highly 
developed  plants  and  animals  may  remain  connected  together 
throughout  life  by  thh?  strands  of  protoplasm.  We  have  already 
noticed  an  example  otf  this  continuity  of  the  protoplasm  in  the 


CONTINUITY  OF  LIFE 


67 


case  of  Volvox  ;  another  is  shown  in  Fig.  28,  B,  which  represents 
a  syncytial  epithelium. 

On  the  other  hand,  in  some  of  the  lower  organisms  —  the 
Myxomycetes,  slime-fungi  or  Mycetozoa,  as  they  are  variously 
called  —  numerous  originally  separate  Amoeba-like  individuals 
may  fuse  together  to  form  plasmodia,  which  may  continue  to 
feed  and  grow  and  undergo  nuclear  division  until  they  form  great 
sheets  of  living  protoplasm  containing  perhaps  hundreds  or 
thousands  of  nuclei  (Fig.  29).  These  and  similar  facts,  how- 
ever, interesting  and 
instructive  as  they  un- 
doubtedly are,  cannot 
be  regarded  as  con- 
stituting a  serious 
invalidation  of  the  cell 
theory. 

There  is  no  more 
fundamental  or  more 
stimulating  conception 
in  the  domain  of  bio- 
logical science  than 
that  of  the  continuity 
of  life  as  formulated  by 
this  theory.  We  have 
to  imagine  the  whole 

Organic  WOrld    as    C011- 
.  . 

sisting  of  a  continuous 

cfroarn     nf    liin'no-    rivr. 
living    pl( 

toplasm,    which   com- 

,    .       n 

menced  to  now  many 
millions  of  years  ago  and  has  continued  without  interruption 
ever  since.  At  every  cell-division  the  stream  branches  and 
physical  continuity  is  more  or  less  completely  interrupted,  but 
this  in  no  way  invalidates  the  conclusion  that  if  all  living  things 
did  not  actually  have  a  common  origin  in  a  single  primordial 
protoplasmic  unit,  they  probably  at  least  originated  from  several 
such  units  which  themselves  arose  under  unknown  conditions 
from  inorganic  matter. 

The  modern  science  of  cytology,  which  is  contrasted  with 
histology  as  the  study  of  individual  cells  rather  than  that  of 
tissues  or  cell  combinations,  and  which  is  yielding  such  important 

F  2 


FIG.  28. 

A  single-layered  epithelium  with  very  distinct  cell- 
outlines,  from  the  brain  of  a  reptile  (Sphenodon 

punctatus),  x  750. 

A  syncytial  epithelium,  without  cell-outlines,  from 
another  part  of  the  brain  of  the  same  animal 

x  750. 

nu.  nuclei. 


68         OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

results  both  for  pure  biology  and  for  medical  science,  owes  its 
origin  and  development  entirely  to  the  elaboration  of  the  cell 


FIG.  29. — Part  of  the  Plasmodiura  of  a  Mycetozoon  (Badhamia  utricularis), 

X  50.     (From  a  photograph.) 
The  small  dark  spots  are  the  very  numerous  nuclei. 

theory  under  the  influence  of  improved  methods  of  microscopical 
investigation. 

There  is  another  point  of  view  with  regard  to  the  living  cell 
which  we  may  briefly  refer  to  in  this  place.  It  is  not  only  the 
morphological  or  structural  unit  of  the  body  but  also  the  physio- 
logical or  functional  unit.  All  the  essential  vital  processes  take 
place  within  the  bodies  of  cells,  and  all  the  different  materials 
required  for  and  produced  by  these  processes  pass  in  and  out  of 
them.  Each  cell  maybe  looked  upon  as  a  microscopic  laboratory 
in  which  the  complex  chemical  reactions  comprised  under  the 
term  metabolism  take  place,  and  although,  in  the  higher 
organisms,  the  cells  have  become  mutually  dependent  upon  one 
another,  yet  each  retains  a  certain  degree  of  physiological  as 
well  as  of  morphological  individuality. 


CHAPTER  VI 

The  multiplication  of  cells — Mitotic  and  amitotic  nuclear  division. 

WE  have  already  seen  that  the  possibilities  of  structural 
differentiation  within  the  limits  of  the  individual  cell  are  by  no 
means  exhausted  by  the  distinction  between  cytoplasm  and 
nucleus,  but  it  is  only  when  we  come  to  study  in  detail  the  pro- 
cess of  cell- division  that  we  begin  to  gain  any  adequate  concep- 
tion of  the  fundamental  complexity  of  the  organic  unit.  We 
have  hitherto  spoken  of  this  process,  as  it  occurs  for  example  in 
Amoeba,  as  though  it  were  a  simple  matter,  initiated  by  constric- 
tion of  the  nucleus  into  two  parts  and  concluded  by  a  correspond- 
ing division  of  the  cytoplasm.  The  researches  of  the  last  forty 
years,  however,  rendered  possible  by  the  improvements  in 
microscopical  apparatus  and  micro-chemical  technique,  have 
taught  us  that  in  the  vast  majority  of  cases  the  process  of  cell- 
division  is  one  of  extreme  complexity,  accompanied  by  remark- 
able phenomena  which  reveal  a"previously  unsuspected  degree  of 
structural  differentiation  within  the  nucleus  itself.  To  these 
phenomena  Schleicher  in  1878  gave  the  name  karyokinesis,  for 
which  Flemming,  in  1882,  proposed  to  substitute  mitosis.  Both 
these  terms  are  still  in  common  use. 

In  a  typical  cell  (Fig.  30)  the  cytoplasm  (cyt.)  is  a  semi-liquid 
substance  usually  enclosed  in  a  thin  cell-membrane  (animal  cells) 
or  a  thicker  cell- wall  (plant  cells).  It  exhibits  a  microscopic 
structure  which  is  variously  interpreted  as  reticular  (fibrillar), 
alveolar  (foam-like),  or  simply  granular,  the  probability  being  that 
the  real  truth  is  expressed  by  a  combination  of  these  different 
views.  It  may  or  may  not  contain  plastids  of  various  kinds  (e.g. 
chloroplastids  in  green  plants). 

The  nucleus  (nu.\  consisting  of  the  so-called  nucleoplasm  or 
karyoplasm,  is  usually  a  spherical,  more  or  less  centrally  situated 
body  enclosed  in  a  definite  nuclear  membrane  (n.m.).  Within 
this  membrane  the  karyoplasm  is  differentiated  into  various  con- 
stituents. In  the  first  place  there  is  a  network  or  reticulum  of 


70          OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

delicate  protoplasmic  threads,  known  as  the  linin  network,  the 
meshes  of  which  are  rilled  with  a  clear,  probably  liquid  ground- 
substance  known  as  nuclear  sap.  These  two  together  seem  to 
differ  but  little  from  the  cytoplasm  which  lies  outside  the 
nucleus. 

The  most  characteristic  constituent  of  the  nucleus  is  another 
substance,  to  which,  on  account  of  the  readiness  with  which  it 
becomes  coloured  by  certain  dyes,  the  name  chromatin1  has  been 
given.  This  substance  usually  occurs  in  the  form  of  small 
granules  (ch.g.)  scattered  over  the  linin  network,  so  that  when 
very  close  together  they  appear  to  form  a  chromatin  network, 


Hllcf  . 


cyt. 


FIG.  30. — Diagram  01  a  typical  Cell. 

ch.g.,  chromatin  granules;  c.m.,  cell-membrane;  c\s.,  centrosonies  lying  in  centrosphere ; 
cyt.,  cytoplasm  ;  n.in.,  nuclear  membrane;  nu. ,  nucleus  ;  nucL,  nucleolus. 

while  not  infrequently  a  specially  large  aggregation  of  chromatin 
substance  forms  a  nucleolus  or  karyosome  (nucL). 

Owing  to  the  presence  of  this  chromatin  the  nucleus  as  a  whole, 
under  low  powers  of  the  microscope,  appears  to  be  deeply  coloured 
by  such  stains  as  various  preparations  of  logwood  and  carmine 
and  the  basic  aniline  dyes.  We  have  already  had  occasion  to 
observe  this  staining  property  in  the  case  of  the  hair-cells  of 
Tradescantia,  where,  it  will  be  remembered,  the  nucleus  becomes 
deeply  stained  by  the  coloured  cell-sap  as  soon  as  the  cells  are 
killed  by  the  action  of  alcohol.  Other  stains,  again,  affect  the 
cytoplasm  rather  than  the  nucleus,  and  these  various  chemical 
reactions  enable  us  to  differentiate  fairly  sharply  between  the 
different  constituents  of  which  the  cell  is  composed,  though  it  is 
a  matter  of  some  doubt  exactly  how  far  the  structure  of  the 

1  Greek  xP&l*a>  a  colour. 


MITOTIC   DIVISION  OF   CELLS.  71 

living  protoplasm  really  corresponds  to  the  appearances 
exhibited  in  our  preparations. 

The  differences  in  their  staining  reactions  of  course  indicate 
corresponding  differences  in  chemical  composition  between  the 
chrornatin  and  the  cytoplasm,  and  analysis  has  shown  that  the 
chromatin  is  characterized  by  the  presence  of  relatively  large 
quantities  of  phosphorus.  This  is  contained  in .  the  complex 
nucleinic  acid,  with  which  various  albuminous  bodies  may  be 
combined  to  form  the  chromatin  substance. 

When  a  typical  animal  cell  is  about  to  divide  another  structure 
makes  it  appearance,  usually  just  outside  but  occasionally  inside 
the  nucleus.  This  is  the  centrosome  (Fig.  30,  c.s.),  a  very 
minute  body  which  has  peculiar  staining  properties  and  which 
is  surrounded  by  a  Differentiated  area  of  protoplasm  known 
as  the  centrosphere  or  attraction  sphere  (Fig.  31,  A,  csph). 
It  has  been  questioned  whether  a  centrosome  and  attraction 
sphere  are  always  present  or  whether  they  make  their 
appearance  only  when  the  nucleus  is  about  to  undergo 
mitosis.  This  process  certainly  seems  to  be  initiated  by  the 
centrosome,  which  may  divide  into  two  parts  long  before  the 
nucleus  itself  commences  to  do  so,  so  that  two  centrosomes  often 
appear  alongside  the  so-called  resting  nucleus  (Figs.  30 ;  31,  A). 
Presently  the  two  centrosomes  move  away  from  one  another, 
both  still  keeping  close  to  the  nucleus,  and  each  is  now  seen  to  be 
surrounded  by  its  own  attraction  sphere  (Fig.  31,  B).  Around 
each  attraction  sphere  delicate  threads  or  fibrillse  of  the  cyto- 
plasm become  radially  arranged  to  form  a  star  or  aster,  and 
the  rays  of  the  asters  which  lie  between  the  two  centrospheres 
combine  to  form  a  spindle  (Fig.  31,  C,  ksp). 

In  the  meantime  remarkable  changes  have  commenced  in  the 
nucleus  itself.  The  chromatin  granules,  together  with  the  liuiu 
by  which  they  are  apparently  held  together,  have  arranged  them- 
selves in  the  form  of  a  long  coiled  thread,  the  spirerne  (Fig.  31,  B), 
and  presently  the  nuclear  membrane  begins  to  disappear  (Fig.  31, 
C,  km),  being  apparently  dissolved  in  the  general  protoplasm. 
In  this  way  the  distinction  between  cytoplasm  and  nucleoplasm 
is  obliterated. 

The  spireme  thread  breaks  up  into  a  number  of  short 
lengths  known  as  chromosomes  (Fig.  31,  C,  chrs),  the  actual 
number  being,  with  certain  exceptions,  a  constant  character  for 
each  species  of  plant  or  animal.  The  centrosomes  at  about  this 


72          OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

time  take  up  their  positions   at   points  corresponding   to  two 
opposite  poles  of  the  original  nucleus,  with  the  spindle  of  fine 


FIG.  31. — Diagram  of  the  principal  Stages  in  the  mitotic  Division  of  the 
Nucleus  in  a  typical  Animal  Cell.  (From  Weismann's  "  Evolution 
Theory,"  adapted  from  E.  B.  Wilson.) 

aeq,  equatorial  plate ;  chr,  chromatin  ;  chrs,  chromosomes ;  cs,  centrosome ;  csph,  centro- 
sphere,  containing  one  or  two  centrosomes ;  kk,  nucleolus ;  km,  nuclear  mem- 
brane; kn,  nucleus;  ksp,  sp,  nuclear  spindle;  p,  aster;  tk,  daughter  nucleus; 
tz,  daughter  cell ;  ek,  cytoplasm  forming  the  cell  body. 

protoplasmic  fibres  stretched  between  them  (Fig.  31,  D),  and  the 
chromosomes  "go  on  the  spindle,"  arranging  themselves  in  a 
so-called  equatorial  plate  across  its  widest  part  (Fig.  31,  D,  aeq). 


MITOTIC  DIVISION   OF   CELLS  73 

The  shape  of  the  chromosomes  varies  much  in  different  cases  ; 
they  may  be  more  or  less  spherical,  but  they  are  frequently  short, 
rod-like  bodies,  often  shaped  like  a  V  (Fig.  32,  A).  Each  one  is 
composed,  like  the  spireme  thread  from  which  they  are  derived, 
of  an  aggregation  of  chromatin  granules,  held  together  by  a 
linin  basis.  The  chrmnatin-granules  arer  sometimes  arranged 
like  the  beads  on  a  necklace  (Figs.  32,  B ;  77),  and  are  known  as 
chromomeres.  The  number  of  the  chromosomes  also  varies 
greatly,  from  as  low  as  two  in  a  variety  of  the  horse-worm 
(Ascaris)  to  as  many  as  one  hundred  and  sixty-eight  in  the  shrimp 
Artemia.  In  cases  where  the  chromosomes  are  very  small  each  one 
may  perhaps  be  equivalent  to  only  a  single  chromomere. 

Either  before  or  after  taking  up  its  position  in  the  equatorial 

B 


FIG.  32. — Sperm-mother- cells  of  a  Salamander,  during  Mitosis.  In  A  the 
chromosomes  are  shown  ;  in  B  the  spireme  thread  is  split  lengthwise, 
and  also  shows  very  clearly  the  chromomeres  of  which  it  is  made 
up.  (From  Weismann's  "Evolution  Theory,"  after  Hermann  and 
Driiner.) 

c,  dividing  centrosome ;  chr,  chromosomes ;  Jd,  chromomeres ;  zk,  cytoplasm. 

plate,  each  chromosome  splits  longitudinally  into  two  parts 
(Fig.  81,  D),  in  fact  the  splitting  can  sometimes  be  observed  in 
the  spireme  thread  even  before  it  breaks  up  transversely  into 
chromosomes  (Fig.  32,  B).  The  result  of  this  splitting  is  that 
the  number  of  chromosomes  is  doubled ;  but  the  daughter  chromo- 
somes very  soon  separate  into  two  equal  groups,  one  of  which 
moves  towards  each  centrosome  (Fig.  31,  E)/  Each* group  contains 
one  of  £he  two  halves  of  each  parent  chromosome. 

Having  migrated  to  opposite  poles  of  the  spindle  the  two 
groups  of  daughter  chromosomes  there  form  the  foundations  of  two 
new  nuclei  (Fig.  31,  F).  The  chromosomes  break  up  into  granules 
again ;  a  new  nuclear  iriembrane  is  formed,  whereby  a  portion  of 
the  general  cytoplasm  is  separated  off  to  form  the  linin  network 
and  ground-substance  of  the  nucleus  ;  the  asters  and  nuclear 


74         OUTLINES  OF  EVOLUTION ABY  BIOLOGY 

spindle  more  or  less  completely  disappear — though  the  centro- 
some  may  certainly  persist  in  some  cases  if  not  in  all — and  the 
newly  constituted  nucleus  (Fig.  31,  G,  tk)  enters  upon  a  longer  or 
shorter  period  of  inactivity  accompanied  by  growth. 

In  the  meantime  the  cytoplasm  which  constitutes  the  cell-hody 
has  also  divided  into  two  parts  in  a  plane  which  passes  through 
the  middle  of  the  nuclear  spindle  and  at  right  angles  to  its 
length.  In  animal  cells  this  division  is  usually  effected  by  a 
constriction  which  starts  from  the  outside  (Fig.  31,  F,  G)  and 
in  plant  cells  by  the  deposition  of  a  cell-plate  (Fig.  347  ^E>  C-P-) 
in  the  equator  of  the  nuclear  spindle.  This  cell-plate  forms  the 
foundation  of  the  double  cell-wall  which  will  separate  the 
daughter  cells  ;  it  must  not,  of  course,  be  confounded  with 
equatorial  plate  formed  temporarily  by  the  chromosomes. 

Various  attempts  have  been  made  to  explain  the  dynamics  of 
this  remarkable  process  of  mitosis  or  karyokinesis,  the  essential 
features  of  which  are  always  much  the  same  though  the  details] 
vary  considerably  in  different  cases.  The  centrosomes,  with( 
their  centrospheres,  asters  and  spindle,  sometimes  spoken  of 
collectively  as  the  achromatic  figure,  are  usually  regarded  as  a 
special  mechanism  for  bringing  about  the  equitable  partition  of 
the  chromatin  substance  between  the  two  daughter  nuclei.  This 
substance  is  evidently  so  important  that  no  rough  and  ready 
division  will  suffice.  It  is  probable,  as  we  shall  see  later  on,  that 
the  chromomeres  of  which  each  chromosome  is  composed  have 
different  properties,  and  that  it  is  necessary,  in  ordinary  cell- 
division,  not  only  that  the  chromosome  as  a  whole  shall  be 
divided  into  two  parts  but  that  each  daughter  nucleus  shall  have 
its  share  of  each  individual  chromomere  (compare  Fig.  77).  In 
other  words  a  qualitative  as  well  as  a  quantitative  division  of 
the  chromatin  material  has  to  be  effected,  and  this  is  secured  by 
the  longitudinal  splitting  of  the  chromosomes.  A  transverse 
division  would  only  result  in  the  separation  of  the  chromomeres 
into  two  groups,  but  the  longitudinal  division  involves  each  one. 

According  to  some  observers  the  fibres  of  the  nuclear  spindle 
are  actively  contractile  and  actually  pull  the  two  halves  of  each 
split  chromosome  asunder.  Others  maintain  that  the  centro- 
somes attract  the  chromosomes  in  somewhat  the  same  way  as  the 
poles  of  a  horse-shoe  magnet  attract  iron  filings  sprinkled 
between  them. 

Of  late  years  the  electro-magnetic  explanation  has  been  coming 


MITOTIC   DIVISION   OF   CELLS  75 

more  prominently  to  the  front.  Thus  Gallardo  has  suggested 
that  the  chromatin  substance  is  charged  with  negative  and  the 
cytoplasmic  colloids  with  positive  electricity,  while  the  centro- 
somes  are  capable  of  acquiring  a  positive  potential  higher  than 
that  of  the  general  cytoplasm.  Increase  of  this  potential  causes 
the  centrosome  to  divide  and  the  radiations  which  form  the 
asters  and  spindle  indicate  lines  of  force  in  the  cytoplasm.  The 
two  daughter  centrosomes,  inasmuch  as  they  bear  like  charges  of 
electricity,  repel  one  another.  In  a  similar  way  the  chromosomes 
divide  under  the  influence  of  their  high  negative  charges  and 
the  two  halves  of  each  repel  one  another  and  are  at  the  same 
time  attracted  by  the  positive  centrosomes.  The  two  new  groups 
of  negatively  charged  chromosomes  then  attract  the  positive 
cytoplasm  in  opposite  directions  and  thus  the  division  of  the  cell 
body  follows  upon  that  of  the  nucleus. 

Whatever  may  be  the  physical  explanation  of  these  complex 
phenomena,  we  must  think  of  them  as  lying  at  the  root  of  all 
normal  processes  of  growth  and  multiplication  in  the  higher 
plants  and  animals.  With  comparatively  rare  exceptions,  some 
of  which  will  be  mentioned  later  on,  every  one  of  the  innumer- 
able series  of  cell-divisions  initiated  by  the  fertilized  ovum, 
and  continued  throughout  life  in  the  growth  and  repair  of  tissues, 
is  accompanied  by  complicated  processes  similar  to  those  above 
described.  The  process  of  cell-multiplication,  however,  is 
frequently  confined  in  adult  organisms  to  certain  regions. 
Thus,  as  we  have  already  seen,  in  the  higher  animals  the  growth 
of  the  epidermis  depends  upon  cell-divisions  which  go  on  only  in 
its  deepest  layer,  the  stratum  Malpighii  (Fig.  18,  s.m.).  Most  of 
the  cells  in  the  body  sooner  or  later  lose  the  power  of  division, 
but  they  are  then  usually  short-lived,  as  in  the  case  of  those 
cells  which  form  the  outer  layers  of  the  epidermis  and  which 
rapidly  become  converted  into  more  or  less  horny  scales  to  be 
cast  off  on  reaching  the  surface. 

The  majority  of  the  tissues  are  thus  renewed  throughout 
life  by  the  mitotic  activity  of  some  unspecialized  cell-group,  a 
high  degree  "of  specialization  in  the  tissue  cells  of  the  higher 
organisms  being  always,  as  we  have  already  seen  in  the  case  of 
red  blood  corpuscles  and.  nerve  cells,  accompanied  by  the  loss  of 
the  power  of  multiplication.  ^ 

The  limitation  of  cell-multiplication  to  definite  circumscribed 
regions  of  the  body  is  perhaps  best  seen  in  the  case  of  the  higher 


76 


OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


plants,  where  the  various  meristematic  or  actively  dividing 
tissues  remain  in  an  undifferentiated  embryonic  condition  and 
give  rise  to  those  additions  to  the  permanent  tissues  whereby 
growth  is  effected.  Such  actively  dividing  meristem  is  found  at 
the  growing  points  of  stems  and  roots,  where  it  serves  to  bring 
about  growth  in  length,  and  in  the  cambium,  which  serves,  by 
the  addition  of  new  elements  to  the  wood  and  the  bast,  to  bring 
about  growth  in  thickness. 

The  microscopic   appearance  of   such   a  meristematic  tissue, 


B. 


FIG.  33.— Part  of  a  longitudinal  Section  of  the  actively  growing  Boot  of  a 
Hyacinth  (Qaltonia  candicans}  showing  the  Nuclei  of  the  Cells  in  various 
stages  of  mitotic  Division,  A  X  280 ;  B  X  640.  (From  photographs.) 

when  suitably  stained  and  prepared  for  examination,  is  shown  in 
Fig.  33,  taken  from  photographs  of  part  of  a  longitudinal  section 
of  the  growing  point  of  the  root  of  a  hyacinth  (Galtonia  candicans). 
The  cell-walls  are  as  yet  thin  and  inconspicuous  and  filled  with 
dense  protoplasm,  while  the  conspicuous  nuclei  exhibit  all  stages 
of  mitosis,  the  whole  forming  a  striking  contrast  to  the  dead 
tissues,  such  as  cork  and  wood,  of  which  the  bulk  of  many  plants 
is  made  up,  and  which  consists  merely  of  cell-walls  without  any 
protoplasmic  contents  (cf.  Figs.  6  and  7). 

Mitosis  in  the  cells  of  the  higher  plants  is  usually,  though  by 


MITOSIS  IN  PLANT  CELLS  77 

no  means  always,  characterized  by  the  absence  of  recognizable 
centrosomes.  The  actual  appearance  of  some  of  the  principal 
stages  in  the  process  is  shown  more  highly  magnified  in  Fig.  34, 

A. 


Nuls 


-cp 


D.  E.  F. 

FIG.  34. — Six  selected  Stages  in  the  mitotic  Division  of  the  Nucleus  in  the 
growing  Boot  of  Galtonia  candicans^  X  1120.     (From  photographs.) 

A.  Resting  nucleus  with  large  nucleolus  (Nuls.). 

B.  Spireme  stage,  with  coiled  dire-matin  thread. 

C.  The  spireme  thread  has  broken  up  into  chromosomes  which  are  forming  the  equa- 

torial plate. 

D.  The  chromosomes  have  split  longitudinally  and  the  two  groups  of  daughter  chromo- 

somes thus  formed  are  passing  to  opposite  poles  of  the  spindle. 

E.  Formation  of  the  cell-plate  (c.p.)  across  the  equator  of  the  nuclear  spindle. 

F.  Completion  of  the  cell-division,  and  disappearance  of  the  individual  chromosomes  in 

the  daughter  nuclei. 

which  represents  a  series  of  six  selected  stages  arranged  in  proper 
sequence,  reproduced  from  photo -micrographs. 

Fig.  34,  A  represents  the  so-called  resting  stage  of  the  nucleus, 
in  which  it  will  be  noticed  that  there  is,  in  addition  to 
the  minute,  scattered  chromatin  granules,  a  large  spherical 
chromatin  nucleolus  orkaryosonie  (Nuls.).  B  shows  the  spireme 


78 


OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


stage,  with  the  chromatin  granules  collected  together  in  a  long 
spirally  coiled  thread  and  the  nucleolus  still  very  conspicuous. 
C  shows  the  group  of  chromosomes  formed  by  transverse  breaking 


FIG.  35. — Mitosis  in  the  segmenting  Egg  of  the  Horse-Worm  (Ascaris  megalo- 
ceplmla),  X  770.     (From  photographs.) 


tindle  (sp.)t 
|jng  out  of 
'  ig  from 

'(chr.) 


A.  Lateral  view  of  the  egg  during  the  first  cleavage;  showing  the  nuclear 

the  equatorial  plate  (aeq.),  one  of  the  two  centrosomes  (c.s.),  the  other 
focus,  and  the  asters  (as.)  formed  by  fine  threads  of  protoplasm  ra  " 
around  the  centrosomes.  Polar  bodies  (p.b.)  are  also  shown. 

B.  The  same  stage  viewed  from  one  pole,  showing  the  four  V-shaped  chronics 

in  the  equatorial  plate. 

C.  The  first  division  is  completed  and  the  nuclei  have  again  passed  into  the  spireifte  stage. 

A  polar  body  (p.b.)  is  still  visible.  i 

D.  Each  of  the  first  two  blastomeres  has  again  reached  the  stage  represented  in  A 

and  B. 

up  of  the  spireme  thread.  The  karyosome  has  now  disappeared, 
having  apparently  been  used  up  in  the  formation  of  the  chromo- 
somes. D  shows  the  two  groups  of  daughter  chromosomes 
formed  by  longitudinal  splitting  of  the  parent  chromosomes  and 
retreating  towards  the  two  ends  of  the  spindle,  which  is  only 


MITOSIS   IN   ANIMAL   CELLS  79 

.dintly  visible.  E  shows  the  commencement  of  the  cell-plate 
(c.p.)  across  the  middle  of  the  spindle,  and  F  the  two  young 
daughter  cells  each  with  a  new  nucleus  in  which  the  chromo- 
somes have  again  broken  up  into  granules. 

It  is  only  by  the  examination  of  large  numbers  of  examples 
that  all  the  minute  details  of  the  process  can  be  elucidated,  but 
the  main  features  as  represented  in  the  above  figures  can  very 
easily  be  made  out. 

Eor  comparison  with  the  process  of  mitosis  as  seen  in  typical  / 
plants  such  as  Galtonia,  we  may  take  the  first  division  of  the 
fertilized  egg  in  the  horse-worm,  Ascaris,  a  classical  subject  from 
the  study  of  which  much  of  our  knowledge  of  nuclear  division  in 
animal  cells  has  been  derived.  In  this  case  there  are  only  four 
chromosomes,  but  they  are  large  and  conspicuous,  and  charac- 
teristically V-shaped  when  forming  the  equatorial  plate  on  the 
spindle. 

Fig.  35  is  again  taken  from  actual  photographs.  In  this 
figure,  A  represents  a  side  view  of  the  entire  egg*°"'l  during 
the  division  of  the  nucleus,  with  spindle  (sp.),  asters  (as.), 
centrosomes  (one  only  of  which,  cs.,  appears  in  the  photograph, 
the  other  being  out  of  focus),  and  equatorial,  plate  (aeq.).  B 
shows  a  similar  stage  viewed  from  .pne  pole,  so  that  the  spindle 
itself  does  not  appear,  but  the  four  chromosomes  forming  the 
equatorial  plate  are  distinctly  visible.  C  shows  the  two  daughter 
cells  or  blastomeres  resulting  from  the  first  division  of  the  egg, 
each  with  the  nuclHls  preparing  for  further  division,  and  D 
represents  a  latertyfeagt  Sji  twlrich  the  nucleus  of  each  daughter 
cell  is  again  actual]}*  ia'  process  of  division  and  shows  the 
separate  chromospmes  very1  distinctly. 

It  must  not  "be  supposed  tiat  the  phenomena  of  mitosis  are  by 
any  means  confined  to  the  higher  animals  and  plants  ;  they  are 
observable  throughout  the  animal  and  vegetable  kingdoms,  in 
unicellular  as  well  as  multicellular  forms.  The  process  has  long 
been  known  to  take  place,  for  example,  in  at  any  rate  some 
Amoebae,  and  it  probably  occurs  wherever  there  is  a  clearly 
differentiated  nucleus.  The  Bacteria  and  their  allies,  in  which 
the  chromatin  granules  are  scattered  throughout  the  cell  body 
and  there  is  no  proper  differentiation  into  cytoplasm  and  nucleus, 
apparently  form  exceptions  to  the  general  rule. 

There  are, however,  even  amongst  the  higher  animals,  some  cases 
of  cell-division  which  do  not  exhibit  mitotic  phenomena,  but  in 


[VOLUTION ABY  BIOLOGY 

the-  nucleus  appears  simply  to  constrict  into  two  or  more 
rts  (Fig.  36) .  This  is  known  as  direct  or  amitotic  nuclear  division. 

It   is   frequently   met   with    in 
degenerating   cells   and  patho- 
logical tissues,  but  it  is  doubtful 
if  it  ever  occurs  (in  the  higher 
organisms  at  any  rate)  in  cells 
which  are  destined  to  undergo 
bng  -  continued   multiplication., 
We  may  therefore  regard-it  as 
a  more  or  less  abnormal  proc1 
with  which  we  have  no  ne 
cern  ourselves  any  further.     The  phenomena  of  mitosis,  on 
Her  hand,  are  thorough  y  normal  and  practically  universal, 
,3  wo  shall  see  later  on,  they  are  of  the  deepest  significance 
the  point  of  view  of  the  theories  of  heredity  and  variation. 


;;G. — Ainitotic  imcleiu  Division 
as  seen  in  Cells  from  the  Cavity 
of  the  Paraph ysis  in  Sphenodon, 
X  1000, 


nil.  nuclei. 


i 


PART  II.— THE   EVOLUTION   OP   SEX 


CHAPTEE  VII 

Limitation  of  the  powers  of  cell-division — Rejuvenescence  by  conjugation 
of  gametes — The  origin  of  sex  in  the  Protista. 

BY  the  process  of  cell-division  an  unbroken  continuity  has 
been  established  in  the  chain  of  living  things  from  the  earliest 
appearance  of  unicellular  organisms  to  the  present  day.  Every 
cell  is  the  descendant  of  pre-existing  cells  and,  in  accordance 
with  the  theory  of  evolution,  all  cells  which  exist  to-day,  distri- 
buted amongst  the  bodies  of  countless  millions  of  different 
organisms,  could,  if  our  knowledge  were  sufficiently  complete,  be 
traced  back  to  a  single  ancestral  cell. 

It  by  no  means  follows  from  these  considerations,  however, 
that  there  is,  under  natural  conditions,  no  limit  to  the  ordinary 
process  of  cell-division.  On  the  contrary  it  is  well  known  that 
in  any  cell  family,  whether  belonging  to  a  unicellular  or  a  multi- 
cellular  organism,  the  power  of  multiplication  tends  t( 
exhausted,  and,  if  that  particular  cell  family  is  to 
existence,  has  to  be  in  some  way  renewed. 

Take,  for  example,  an  ordinary  ciliate  or  flagellate  Prot<f!z 
which  multiplies  by  simple  fission.  If  ^a  single  individual  be 
isolated  and  placed  in  water  containingjjfctiable  food  materi^, 
and  kept  under  suitable  conditions  of  temperature,  light  and  so 
forth,  it  will  multiply  very  rapidly,  until  possibly  hundreds  of 
generations  of  separate  cells  have  been  produced  and  the  total 
number  increased  perhaps  to  millions.  But  under  ordinary 
circumstances  a  time  presently  arrives  when  the  individuals 
begin  to  show  signs  of  exhaustion,  accompanied  by  physical 
degeneration,  and  to  slack  off  in  their  rate  of  multiplication. 
They  may  be  stimulated  to  renewed  activity  for  a  time  by  special 
feeding  or  by  constantly  varying  the  culture  medium,1  but  in  a 

1  Mr.  L.  L.  Woodruff  has  kept  a  culture  of  Paramoecium  under  observation  for 
five  years  (to  May,  1912),  taking  precautions  to  prevent  the  possibility  of  con- 
jugation, but  constantly  varying  the  culture  medium.  During  this  time  more  than 
three  thousand  generations  of  Paramoecium  were  produced  by  repeated  fission — an 

G 

ii.  .  •    •'•i-        " 


OUTLINES  OF  EVOLUTIONARY  BIOLOGY 


state  of  nature  the  chief  if  not  the  only  means  by  which  the 
family  can  be  kept  from  speedy  extinction  is  conjugation,  the 


4- 

FIG.   37. — Life  History  of  Copromonas.      (From  Bourne's  "Comparative 
Anatomy,"  after  Dobell.) 

cv.,   contractile    vacuole ;    cph.,   cell  pharynx ;    cst.,  cell  mouth ;   fv.,   food  vacuole 
N.,  nucleus;  B.,  reservoir;  tr.,  flagellum.     (For  further  explanation  see  text.) 

exhausted  individuals  approaching  one  another  and  finally  uniting 
in  pairs.     In  this  way  they  appear  to  become  rejuvenated  and 

average  of  about  one  division  every  fifteen  hours — and  at  the  end  of  the  period  the 
organisms  were  still  in  a  perfectly  normal  condition. 


LIFE   HISTORY  OF   COPROMONA8  88 

their  failing  powers  of  multiplication  by  cell-division  are  com- 
pletely restored. 

For  the  purpose  of  studying  this  process  of  conjugation  in  its 
primitive  simplicity  we  can  hardly  do  better  than  take  the  minute 
flagellate  form  Copromonas,  which  is  found  in  water  frequented 
by  frogs,  from  the  excrement  of  which  it  derives  its  nutriment. 
The  adult  organism  (Fig.  37,  A)  consists  of  a  very  minute  ovoid 
mass  of  protoplasm  with  a  single  flagellum  (tr.)  springing  from 
the  narrow  end.  Alongside  the  base  of  the  flagellum  is  a  definite 
cell  mouth  (cytostome,  cst.)  through  which  solid  particles  of  food 
are  taken  into  the  interior  of  the  body.  Close  to  this  there  is 
a  contractile  vacuole  (cv.)y  accompanied  by  a  "reservoir"  (72) 
into  which  it  discharges.  The  nucleus  (N)  is  situated  nearer  to 
the  broadly  rounded  hinder  end  of  the  body,  which  may  also 
contain  a  number  of  food-vacuoles  (fv.). 

If  the  food  supply  be  abundant  the  individual  Copromonas  will 
grow  and  presently  divide  into  two  by  simple  longitudinal  fission, 
which  commences  at  the  narrow  anterior  end  (Fig.  37,  B — D). 
The  division  of  the  nucleus  is  said  to  be  amitotic.  The  two 
daughter  cells, separate,  feed,  grow  and  repeat  the  process,  and  in 
this  way  a  whole  swarm  of  monads  is  produced.  In  the  course  of  a 
few  days,  however,  they  appear  to  become  exhausted  and  conjuga- 
tion sets  in,  the  individuals  uniting  in  pairs  (Fig.  37,  2 — 5).  The 
result  of  each  such  union  is  a  single  larger  individual,  which  may 
either  undergo  a  period  of  rest  within  the  protection  of  a  special 
envelope  or  cyst  (Fig.  37,  7),  or  at  once  assume  the  ordinary 
form  and  begin  to  multiply  with  renewed  activity  (Fig.  37,  V). 
F.or  the  continued  existence  of  the  species  it  is  probably  necessary 
that  the  encysted  monads  should  at  some  time  or  another  be 
swallowed  by  frogs  and  passed  out  again  in  the  faeces,  in  order 
that  they  may  be  brought  in  touch  with  the  necessary  food 
supply. 

We  have  here,  as  in  the  case  of  H&matococcus  described  in 
Chapter  III.,  a  perfectly  typical  example  of  conjugation1  occurring 
;tt  longer  or  shorter  intervals  in  the  life  cycle  of  the  organism.  The 
whole  process  consists  in  the  union  of  two  separate  cells,  known 
in  this  connection  as  gametes, 'to  form  a  single  cell  known  as  the 
zygote,  and  it  is  of  the  utmost  importance  to  observe  that  not  only 
is  there  a  union  between  the  cytoplasm  of  the  two  gametes  but 
the  nuclei  also  unite  to  form  a  single  zygote  nucleus.  Indeed, 

1  Also  known  as  syngamy  or  zygosis. 

Q   2 


84         OUTLINES   OF  EVOLUTIONAEY  BIOLOGY 

as  we  shall  see  later  on,  it  is  the  union  of  the  nuclei  which  is 
the  really  important  part  of  the  business,  for  in  some  cases  (e.g., 
Paramoecium)  the  union  of  the  two  cell  bodies  is  a  merely 
temporary  affair,  a  necessary  preliminary  to  an  exchange  and 
subsequent  union  of  nuclei. 

In  such  simple  cases  as  that  of  Copromonas  we  see  all  the 
essential  features  of  the  sexual  process  which  occurs  so  constantly 
throughout  the  animal  and  vegetable  kingdoms.  It  is  evident 
that  in  itself  conjugation  is  not  a  process  of  reproduction,  for  its 
immediate  result  is  to  halve  the  total  number  of  cells  instead  of 
doubling  it.  It  has  in  fact  exactly  the  opposite  effect  to  that  of 
cell-division.  It  is  a  process  which  appears  to  be  necessary  for 
the  rejuvenescence,  at  longer  or  shorter  intervals,  of  exhausted, 
cells,  whereby  they  are  endowed  with  renewed  powers  of  multi- 
plication by  ordinary  cell-division.  At  the  same  time  it  forms 
the  starting  point  of  all  those  remarkable  structural  modifications 
of  the  organism,  whether  unicellular  or  multicellular,  which 
accompany  the  evolution  of  sex. 

In  Copromonas  and  in  Haematococcus,  although  there  is  a 
true  sexual  process,  there  is  apparently  no  sexual  differentiation 
at  all ;  there  is  no  distinction  between  male  and  female  gametes  ; 
the  two  conjugating  cells  are  exactly  alike,  and  the  conjugation 
is  therefore  said  to  be  isogamous.  In  Copromonas,  moreover,  the 
gametes  or  sexual  cells^are^lndistinguishable  from  the  ordinary 
individuals,  every  individual  being  at  least  a  potential  gamete. 
Starting  from  such  a  case  as  this  we  find,  even  amongst  the 
unicellular  plants  and  animals,  every  stage  in  the  evolution  of  highly 
specialized  male  and  female  gametes,  differing  widely  from  the 
ordinary  individuals  and  from  each  other.  Conjugation  will  then 
take  place  between  two  dissimilar  gametes,  and  is  said  to  be 
anisogamous. 

The  first  hint,  so  to  speak,  of  sexual  differentiation  is.  to* 
be  observed  in  the  behaviour  of  the  conjugating  gametes;  it  is 
a  physiological  rather  than  a  structural  or  morphological 
phenomenon,  and  consists  in  the  fact  that  one  gamete  is  active 
while  the  other  remains  comparatively  passive.  We  shall  find 
that  this  distinction  lies  at  the  root  of  all  sexual  differentiation 
throughout  the  animal  and  vegetable  kingdoms.  The  more  active 
gamete  is  spoken  of  as  male  and  the  more  passive  as  female.  The 
passivity  of  the  female  is  intimately  associated  with  and  probably 
lio  a  large  extent  dependent  upon  the  fact  that  it  contains  more 


I/ 

SEXUAL   DIFFERENTIATION  85 

cytoplasm  and  is  therefore  more  heavily  weighted  than  the  male 
gamete.  This  cytoplasm,  moreover,  is  in  many  cases  densely 
charged  with  food  material,  which  constitutes  the  capital  with 
which  the  zygote,  formed  by  the  union  of  the  two  gametes,  has 
to  begin  its  new  life  cycle. 

It  is  quite  clear  that  the  primary  distinction  between  the  sexes 
is  a  simple  case  of  division  of  labour  accompanied  by  a 
corresponding  structural  differentiation.  Two  ends  have  to  be 
secured  by  the  gametes.  They  must  come  together  in  order  that 
they  may  conjugate,  and  therefore  one  or  both  must  be  capable 
of  active  locomotion.  They  must  also  contain  between  them 
sufficient  material,  either  in  the  form  of  actual  protoplasm  or  of 
some  substance  that  can  easily  be  worked  up  into  protoplasm,  to 
give  the  new  individual  which  results  from  their  union  a  fair 
start  in  life. 

A  cell  body  heavily  weighted  with  food  material  is,  however, 
clearly  incompatible  with  great  activity,  so  one  of  the  two  gametes 
remains  unencumbered  and  becomes  specialized  as  the  active 
partner,  charged  with  the  duty  of  seeking  out  its  mate 
and  bringing  about  their  union,  while  the  other,  more  or  less 
burdened  with  the  necessary  supplies,  passively  awaits  the  event. 
The  conjugation  of  such  differentiated  gametes  leads  to  a  more 
satisfactory  result  than  can  be  attained  in  cases  of  isogamy 
like  that  of  Copromonas,  for  the  new  individual  will  have  a  better 
chance  in  life  owing  to  the  greater  amount  of  capital  with  which  it 
commences.  Such  sexual  differentiation  of  the  gametes  finds  its 
most  complete  expression  in  the  formation  of  female  ova  and 
male  spermatozoa,  which  are  especially  characteristic  of  the 
higher  animals  (compare  Fig.  69),  though  they  also  occur  in 
many  plants  and  even  in  some  unicellular  forms.  The  process 
of  conjugation  in  such  a  case  is  often  spoken  of  as  the  fertiliza- 
tion of  the  ovum  by  the  spermatozoon. 

These  considerations  enable  us  to  understand  at  once  the  great 
difference  in  size  which  usually  distinguishes  the  male  from  the 
female  gamete,  whence  the  general  terms  microgametes  and  mega- 
gametes  so  often  applied  to  them.  The  microgamete  is  as  small 
as~possible  in  order  that  its  activity  may  not  be  impaired ;  the 
megagarnete  is  swollen  out  with  nutrient  material. 

We  may  illustrate  these  general  principles  by  a  brief  description 
of  a  few  more  cases  of  conjugation  amongst  unicellular  organisms. 

Bodo,  or  Heteromita  (Fig.  38),  is  a  very  minute  flagellate  monad 


88         OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


FIG.    38. — Life  History  of  Bodo,    or  the   Springing  Monad,  very  highly 
magnified.     (From  Dallinger  and  Drysdale.) 

A.  Ordinary  individual. 

B.,  C.  Multiplication  by  longitudinal  fission  (the  nucleus  divides  and  the  cell  body  aiid 

both  flagella  split  lengthwise). 
D — F.  Multiplication  by  transverse  fission  (the  nucleus  and  cell  body  divide,  the  trailing 

flagellum  splits  lengthwise  and  a  new  anterior  flagellum  is  budded  out  at  one  end). 
G.  Two  gametes  about  to  conjugate. 
H.  Conjugation. 

J.    Zygote  formed  by  conjugation,  with  flagella  still  attached. 
K.  Fully  formed  zygote. 

L.   Escape  of  spores  by  rupture  of  the  zygote  wall. 
M.   Development  of  the  spores. 
a./.,  anterior  flagellum;    a'./.,  new  anterior  flagellum   sprouting  out;    nu.,   nucleus; 

sp.,  spores  ;  t.f.,  trailing  flagellum  ;  zyg.nu.,  zygote  nucleus. 


LIFE   HISTORY  OF   BODO  87 

which  occurs  in  long-standing  infusions  of  cod's  head.  It  differs 
from  Copromonas  chiefly  in  the  possession  of  two  flagella  and  in 
the  absence  of  a  cell  mouth,  all  its  food  being  taken  in  in  a  state  of 
solution  by  diffusion  through  the  thin  cell  membrane.  The  two 
flagella  both  spring  from  the  beak-like  anterior  extremity.  One 
(A,  a.f.)  extends  forwards  and  by  its  movements  enables  the 
organism  to  swim  actively  about,  the  other  (A,  t.f.)  hangs  down 
and  is  trailed  behind  during  active  locomotion.  The  monad 
anchors  itself  by  the  trailing  flagellum  and  then,  by  coiling  and 
uncoiling  the  latter,  executes  characteristic  springing  movements. 
Asexual  reproduction  (i.e.,  reproduction  without  any  sexual 
process)  is  effected  by  simple  fission,  which  may  be  either 
longitudinal  (B,  C)  or  transverse  (D — F). 

There  are  no  structurally  differentiated  gametes  or  sexual 
cells,  but  conjugation  (G — J)  is  effected  between  two  apparently 
similar  individuals  which  are  indistinguishable  from  the  ordinary 
form.  It  is  noteworthy,  however,  that  one  of  the  two  gametes  at  the 
time  of  union  is  anchored,  while  the  other  swims  actively  up  to  it, 
and  thus  we  get  a  slight  indication  of  physiological  differentiation 
into  active  and  passive,  or  male  and  female.  The  male  gamete 
also  arises  by  a  somewhat  peculiar  method  of  fission.  Conjugation 
of  the  two  gametes  produces  a  zygote  which  has  somewhat  the 
shape  of  a  triangular  sac  (K).  The  flagella  disappear  and  in  the 
interior  of  the  sac  cell-division  goes  on  with  great  rapidity,  giving 
rise  to  an  immense  number  of  very  minute  spores,  which  ultimately 
escape  from  the  corners  of  the  sac  in  the  form  of  very  fine  dust 
(L,  sp.).  Each  spore  no  doubt  is  a  minute  nucleated  cell,  but  it  is 
so  small  that  the  nucleus  cannot  at  first  be  made  out.  It  grows 
by  absorbing  liquid  food  from  the  infusion  in  which  it  lives,  and 
as  it  grows  the  nucleus  becomes  apparent,  flagella  are  put  forth, 
and  the  adult  form  is  gradually  attained  (M).  We  have  here  a 
striking  illustration  of  the  fact  that  the  most  obvious  result  of 
conjugation  is  an  increase  of  the  power  of  cell- division. 

As  a  case  of  complete  morphological  as  well  as  physiological 
differentiation  between  male  and  female  gametes  in  a  unicellular 
organism  we  may  take  that  of  Coccidium  schubergi,  which  occurs 
as  a  parasite  in  the  intestine  of  a  centipede  (Lithobius  forficatus). 
The  life  history  of  this  remarkable  protozoon  is  very  com- 
plicated and  it  is  not  necessary  for  our  purposes  to  describe  it 
in  detail.  The  adult  organisms  occur  in  the  form  of  spherical 
nucleated  cells,  each  actually  inside  one  of  the  epithelial  cells 


88 


OUTLINES   OF   E VOLUTION AEY  BIOLOGY 


which  line  the  intestine  of  the  host  and  upon  which  the  parasites 
feed  (Fig.  39,  A.).  The  latter  increase  in  number  very  rapidly 
by  a  kind  of  multiple  fission,  and  successive  generations  of 
parasites  attack  fresh  epithelial  cells  of  the  host  until  the 
epithelium  is  more  or  less  completely  destroyed.  After  many 
generations  have  been  produced  asexually  in  this  manner  a 
sexual  process  sets  in.  Megagametes  and  microgametes  are  pro- 
duced ;  the  former  by  growth  of  an  ordinary  individual  into  a 
large  spherical  ovum,  or  egg-cell  (Fig.  39,  B.,  ?  gam.)f  the  latter 
by  growth  and  division  of  an  ordinary  individual  into  a  number 


nu.ep. 


c.x 


A.  B. 

FiG.  39. — Coccidium  schulergi,  highly  magnified.     (After  Schaudinn.) 

A.  A  full  grown  Coccidium  lying  within  an  epithelial  cell  of  the  host. 

B.  Male  and  female  gametes  about  to  conjugate. 

coc.,  the  Coccidium ;  c.r.,  the  cone  of  reception  put  out  by  the  female  gamete ;  mi.,  nucleus 
of  female  gamete;  nu. coc., nucleus  of  adult  Coccidium;  nu.ep.,  nucleus  of  epithelial 
cell  of  host ;  £  gam.,  male  gamete  or  spermatozoon ;  £  gam.,  female  gamete  or  ovum. 

of  very  much  smaller  spermatozoa  or  sperm  cells  (Fig.  39,  B., 
$  gam.). 

The  ovum  is  densely  filled  with  food  granules  and  has  no 
power  of  locomotion.  The  spermatozoon  closely  resembles  a 
flagellate  monad,  being  provided  with  a  pair  of  flagella  by  means 
of  which  it  swims  actively  about ;  the  body,  however,  is  long  and 
slender  and  consists  almost  entirely  of  nuclear  (chromatin)  material. 
It  seeks  out  the  ovum,  which  exercises  a  peculiar  attraction  upon 
it,  and  the  two  conjugate,  the  spermatozoon  boring  its  way  into 
the  ovum  and  their  nuclei  fusing  to  form  the  zygote  nucleus. 
The  only  trace  of  activity,  apart  from  nuclear  phenomena,  which 
the  ovum  exhibits  is  the  protrusion  of  a  small  "cone  of  recep- 
tion "  (Fig.  39,  B.  c.r.)  from  the  surface  of  the  cell  towards  the 
approaching  spermatozoon,  which  seems  to  indicate  that  the 


OKIGIN   OF    SEX    IN   PLANTS  89 

attraction  is  mutual.  The  process  of  conjugation  is  followed  as 
usual  by  cell-division  on  the  part  of  the  zygote,  which  in  this 
case  results  in  the  formation  of  a  small  number  of  comparatively 
large  spores,  enclosed  in  tough  protective  envelopes.  From 
these  spores  new  individuals  are  produced  which,  under  favour- 
able circumstances,  commence  the  life  cycle  afresh.  It  is 
extremely  interesting  to  observe  that  we  have  here,  in  a 
unicellular  Protozoon,  as  complete  a  sexual  differentiation  of 
the  gametes  as  we  meet  with  in  any  of  even  the  most  highly 
organized  plants  and  animals. 

We  must  now  briefly  notice  the  sexual  phenomena  exhibited 
by  those  interesting  Protista  which  we  have  already  had  occasion 
to  refer  to  in  Chapter  IV.  under  the  name  Phytoflagellata.  ^  It 
will  be  remembered  that  in  Haematococcus  (Fig.  5)  the  process  of 
simple  fission  sometimes  results  in  the  production  of  a  relatively 
large  number  (32 — 64)  of  small  individuals  instead  of  the  usual  four 
comparatively  large  ones.  These  small  individuals  are  specialized 
gametes,  differing  from  the  large  ones  not  only  as  regards  size 
but  also  in  the  absence  of  the  characteristic  cell-wall  of  the  latter. 
There  is,  however,  no  differentiation  into  male  and  female. 
Conjugation  is  of  the  isogamous  type  and  produces  a  zygote 
which  grows  into  an  ordinary  resting  cell  which  will  presently 
begin  to  multiply  actively  by  ordinary  fission. 

We  have  also  seen  that  this  organism  forms .  the  starting  point 
of  a  series  of  forms,  represented  by  the  genera  Haematococcus, 
Pandorina,  Eudorina  and  Volvox,  which  illustrate  progressive 
stages  in  the  process  of  colony  formation.  The  same  series  also 
shows  us  very  clearly  the  differentiation  between  male  and  female 
gametes — the  origin  of  sex  in  the  vegetable  kingdom.  It  will  be 
remembered  that  Pandorina  forms  colonies  of  sixteen  or  thirty- 
two  cells  enclosed  in  a  common  envelope  (Fig.  10,  A).  Asexual 
multiplication  is  effected  by  each  cell  of  the  colony  dividing  into 
2,  4,  8,  and  finally  16  or  32,  which  form  a  daughter  colony  within 
the  parent,  to  be  liberated  presently  by  softening  of  the  parental 
envelope.  Occasionally,  however,  the  individual  cells  of  a  colony 
divide  each  into  eight  gametes.  These  are  small  cells,  each  with 
a  pair  of  flagella,  which  escape  and  swim  about  separately.  They 
exhibit  no  clear  distinction  into  male  and  female,  but  some  are 
comparatively  large,  some  small,  and  some  intermediate  in  size. 
They  conjugate  in  pairs  (Fig.  10,  B),  and  the  two  members  of  a 
conjugating  pair  are  often,  though  apparently  not  always,  of 


90 


OUTLINES   OF  EVOLUTIONARY   BIOLOGY 


different  siz3S.      It  is  probable   that   we  have  here  a  kind  of 
foreshadowing  of   that  sharp  differentiation  into  large  (female) 


i! 

is 


II     I   3 


£      1 


a  s  .-§ 

i  *  3 

I  .3  -9 

-^  "^  K*> 

•3  I  I 


.a  § s  «!• 


megagametes  and  small  (male)  microgametes  of  which  we  have 
already  spoken. 

In  Eudorina,  which  forms  colonies  somewhat  similar  to  those  of 
Pandorina,  this  differentiation  is  already  fully  expressed  (Fig.  40). 
The  megagametes  or  ova1  differ  scarcely  at  all  from  ordinary 

i  Often  termed  by  botanists  oospheres. 


SEXUAL  DIFFERENTIATION   IN  PROTOPHYTA  •*  91 

^ 
individuals.      They    are    shown    imbedded    in    the    gelatinous 

matrix  of  the  large  colony  on  the  left  of  the  figure.  The  micro- 
gametes  or  spermatozoa,1  on  the  other  hand,  are  much  smaller, 
club-shaped  bodies,  having  a  characteristic  yellowish  colour  and 
with  a  pair  of  flagella  at  the  narrow,  pointed  end  (Fig.  40,  M3). 
They  are  produced  in  bundles  of  sixty-four  by  repeated  divi- 
sion of  a  mother  cell  (Fig.  40,  II — VI).  Thus  the  male  and 
female  gametes,  spermatozoa  and  ova,  do  not  occur  together 
in  the  same  colony,  but  the  colonies,  when  they  consist  of 
gametes  and  not  of  ordinary  individuals,  contain  only  one  or  the 
other  kind.  Hence  the  sexual  differentiation  is  in  this  case 
extended  from  the  gametes  themselves  to  the  colonies  which 
bear  them,  and  we  may  recognize  colonies  of  three  kinds :  (1) 
Asexual,  which  produce  no  gametes  and  reproduce  by  ordinary 
fission  of  all  or  any  of  the  component  cells,  (2)  male  sexual,  which 
produce  microgametes  or  spermatozoa,  and  (3)  female  sexual, 
which  produce  megagametes  or  ova.  The  bundles  or  colonies  of 
microgametes  (Fig.  40,  MI,  Mg)  swim  about  actively  by  means  of 
their  flagella,  apparently  in  search  of  the  larger  and  much  less 
active  female  colonies  (Fig.  40,  I).  Having  found  such  a 
colony  the  now  separated  microgametes  (Fig.  40,  M3)  bore 
tlieir  way  in,  ultimately  conjugating  with  the  megagametes  to 
form  zygotes. 

In  any  one  colony  of  Pandorina  or  Eudorina  the  constituent 
^ells  are  all  of  one  kind,  all  ordinary  asexual  cells  or  all  male 
gametes  or  all  female  gametes.  In  Volvox  (Fig.  11)  we  meet  with 
a  further  advance.  Even  in  asexual  colonies  which  do  not  produce 
gametes  at  all  we  find  the  cells  differentiated  in  so  far  that  some 
only  are  capable  of  giving  rise  (asexually)  to  daughter  colonies, 
while  in  the  female  colonies  only  some  of  the  cells  form  female 
gametes  or  ova  (Fig.  11,  A,  o).  The  male  gametes  (Fig.  11,  B — D) 
are  very  similar  to  those  of  Eudorina.  They  unite  with  the 
female  gametes  to  form  zygotes,  which,  after  a  period  of  rest, 
develop  into  new  Volvox  colonies. 

In  order  to  emphasize  the  fact  that  the  process  of  conjugation 
is  essentially  a  nuclear  phenomenon  we  may  now  turn  to  the 
case  of  Paramoecium.  The  general  appearance  and  structure  of 
this  protozoon  have  been  described  in  Chapter  IV.  (Fig.  8).  It 
multiplies  by  simple  transverse  fission,  and  under  favourable 
conditions  continues  to  do  so  until  exhaustion  sets  in,  when  its 

1  Often  termed  by  ^tanists  spermatozoids. 


92 


OUTLINES   OF  EVOLUTIONAEY  BIOLOGY 


failing  powers  are  restored  by  conjugation.  The  conjugation  in 
this  case,  however,  is  not  quite  like  that  which  takes  place  in 
the  other  unicellular  organisms  which  we  have  been  studying, 
and  the  term  conjugant  may  be  applied,  in  preference  to  the 
term  gamete,  to  the  individuals  concerned. 

It  may  easily  be  observed  that  the  union  of  the  two  con- 
jugants  (Fig.  41)  is  a  merely  temporary  affair.  They  remain 
attached  together  by  their  mouth-bearing  surfaces  for  a  short 


"FiG.  41. — Diagram  of  the  Process  of  Conjugation  in  Paramoecium. 

gam.,  gametic  nuclei;  If.,  mouth;  meg.,  meganucleus;  mic.,  micronucleus ;  mifj. 
migratory  nucleus ;  p.b.,  products  of  the  division  of  the  micronucleus  which  dis- 
appear (polar  bodies);  st.,  stationary  nucleus ;  zyg.,  zygote  nucleus.  (For  further 
explanation  see  text.) 

time  and  then  separate  again  and  continue  their  independent 
lives.  Before  separating,  however,  they  evidently  undergo  some 
kind  of  rejuvenescence  whereby  their  vigour  and  power  of  multi- 
plication are  completely  restored.  This  is  accounted  for  by  the 
fact  that  during  the  time  of  their  union  certain  complex  nuclear 
processes  take  place,  the  net  result  of  which  is  an  exchange  of 
chromatin  material  between  the  two  conjugants. 

It  will  be  remembered  that  Paramoecium  differs  from  most 
Protozoa  in  the  possession  of  two  nuclei,  large  and  small,  or 
meganucleus  and  micronucleus  (Fig.  41,  A,  meg.  and  mic.).  The 


CONJUGATION   IN   PARAMCECIUM  93 

latter  is  alone  concerned  in  the  process  of  conjugation,  the 
meganucleus  in  the  meantime  breaking  up  and  being  absorbed 
into  the  cytoplasm,  to  be  replaced  in  the  manner  described  later 
on .  We  may  confine  our  attention,  therefore,  to  the  behaviour 
of  the  micronucleus.  In  each  conjugant  this  divides  mitotically 
into  two  daughter  nuclei  (Fig.  41,  B,  mic.'  and  p.b.)  each  of  which 
again  divides,  so  that  there  are  now  four  micronuclei  (Fig.  41,  C). 
Of  these  four  three  (#.&.)  go  to  the  bad,  being  apparently  absorbed 
into  the  cytoplasm,  while  the  remaining  one  (mic.")  divides  once 
more,  so  that  each  conjugant  has  now  again  two  micronuclei 
(Fig.  41,  D).  These  two,  though  similar  in  appearance,  differ 
strikingly  in  their  behaviour,  one  of  them  remaining  quiescent 
while  the  other  passes  over  into  the  body  of  the  other  conjugant. 
They  are  therefore  known  respectively  as  the  stationary  (st.)  and 
the  migratory  (mig.)  micronuclei.  In  this  way  the  migratory 
micronuclei  of  the  two  conjugants  change  places  with  one  another, 
as  indicated  by  the  arrows,  and  the  sole  object  of  the  temporary 
union  of  the  two  conjugants  appears  to  be  to  enable  this  inter- 
change to  take  place.  "When  it  has  been  effected  a  true  conjuga- 
tion occurs  'between  the  two  micronuclei  in  each  cell  (Fig.  41,  E, 
gam.),  derived  one  from  each  conjugant.  This  is  the  real  sexual 
process.  The  migratory  and  stationary  nuclei  are  gametic  nuclei 
and  the  result  of  their  union  is  a  zygote  nucleus  (Fig.  41,  F,  zyg.). 
Moreover,  we  have  here  again  an  evident  distinction  into 
male  and  female  gametic  nuclei,  characterized  in  the  usual 
way  by  the  activity  of  the  one  and  the  passivity  of  the  other. 
The  two  conjugants  themselves,  however,  cannot  be  distin- 
guished as  male  and  female,  for  each  produces  both  male 
and  female  gametic  nuclei  and  may  therefore  be  regarded  as 
hermaphrodite. 

After  the  interchange  of  gametic  nuclei  has  taken  place  the  two 
conjugants  separate  as  ex-conjugants  (Fig.  41,  F).  The  zygote 
nucleus  in  each  divides  repeatedly  by  mitosis  and  from  the 
daughter  nuclei  thus  produced  both  micronuclei  and  meganuclei 
are  formed.  Presently  the  ex-conjugants  themselves  begin 
to  divide  once  more  by  fission  and  the  new  micronuclei 
and  meganuclei  are  distributed  amongst  the  new  individuals 
(Fig.  41,  G). 

The  essential  feature  of  this  very  complicated  process  is  clearly 
the  same  as  in  the  simpler  cases  which  we  have  examined,  and 
consists  in  the  union  of  two  nuclei  belonging  to  different  cells  to 


94 


OUTLINES  OF  EVOLUTIONARY  BIOLOGY 


form  a  single  zygote  nucleus  which  has  renewed  powers  of 
multiplication  by  division.  As  already  observed,  the  micronucleue 
alone  takes  part  in  the  process,  the  meganucleus  being  merely 
concerned  in  the  life  of  the  individual  and  its  asexual  multiplication 
by  simple  fission. 


CHAPTEK  VIII 


str. 


Sexual  phenomena  in  multicellular  plants — The  distinction  between  somatic 
cells  and  germ  cells — Alternation  of  sexual  and  asexual  generations — 
Suppression  of  the  gametophyte  in  flowering  plants. 

WHEN  we  consider  the  habit  of  colony  formation  which  is  so 

common  amongst  the  Protophyta,  and  which  we  have  discussed 

in  the   cases  of  Pandorina,  Eudorina  and 

Volvox,  we  see  at  once  that  it  is  impossible  to 

draw  any  strictly  logical  distinction  between 

such   primitive  forms  and  the  true  multi- 
cellular  plants  or  Metaphyta.     The  common 

fresh    water  alga,    Spirogyra,   for  example, 

might  be  regarded  either  as  a  colony  of  single 

cells  or  as  a  very  simple  multicellular  plant 

in  which  the  constituent  cells  exhibit  little 

or  no    differentiation   amongst  themselves. 

In  any  case  it  forms  a  very  con  ;cnient  start- 
ing point  for  the  consideration  of  the  sexual 

phenomena  met  with  in  Metaphyta,  being  in 

this  respect  actually  in  a  much  more  primitive 

condition  than  either  Eudorina  or  Volvox. 
/\yThe  fully  developed  Spirogyra  plants  con- 
/Bist  of    long  green   filaments    of    hair-like 

dimensions,    which    float    in    loose    slimy 

masses  in  clear  fresh  water.     Each  filament 

consists  of  a  single  row  of  cylindrical  cells 
)  placed  end  to  end,  each  cell  being  enclosed  in 

a  thin,  transparent  wall  of  cellulose  (Fig.  42, 
~  c.iv.),  whereby  its  protoplasmic  contents  are 

completely  separated  from  those  of  adjacent 

cells.    The  cytoplasm  forms  a  thin  primordial 

utricle  (p. u.), lining  the  cell-wall  and  enclosing 

a    large  vacuole  (vac.)  filled  with  colourless,  watery  cell-sap,  in 

which  a  more  or  less  central  mass  of  cytoplasm,  containing  the 


FIG.  42. — Part  of  a 
filament  of  Spiro- 
gyra, showing 
one  complete  cell 
and  parts  of  two 
others  ;  highly 
magnified. 

c.w.,  cell-wall;  cr.,  chro- 
matophore  ;  int., 
nucleus  ;  p.u.,  pri- 
mordial utricle; 
pyr.,  pyrenoid;  str., 
strands  of  proto- 
plasm; vac.,  vacuole. 


96 


OUTLINES   OF   EYOLUTIONAEY  BIOLOGY 


nucleus  (nu.),  is.  suspended  by  slender  radiating  protoplasmic 
threads  (sir.).  So  far,  both  in  structure  and  arrangement  of  its 
component  cells,  the  plant  closely  resembles  a  single  hair  of 
Tradescantia  (compare  Fig.  25).  It  differs,  however,  in  the 

presence  in  each  cell  of  one  or  more 
conspicuous  chloroplastids  or  chroma- 
tophores  (cr.),  coloured  bright  green 
by  chlorophyll  and  wound  spirally 
round  and  round  inside  the  cell-wall, 
like  pieces  of  ribbon.  It  is  from 
these  characteristic  structures  that 
the  name  Spirogyra  is  derived. 

The  filaments  increase  in  length  by 
transverse  fission  of  the  component 
cells.  Every  cell,  however,  or  to  speak 
more  accurately  its  protoplasmic  con- 
tents, must  also  be  looked  upon  as 
a  potential  gamete.^Conjugation  in 
some  species  takes  place  between  the 
cells  of  two  filaments  which  are  lying 
side  by  side,  parallel  with  one  another 
(Figs.  43, 44).  In  others  it  takes  place 
between  adjacent  cells  of  the  same 
filament,  but  we  may  confine  our 
attention  to  the  former  case.  The 
first  indication  of  the  process  is  seen 
in  the  formation  of  a  small,  hollow  pro- 
tuberance on  the  wall  of  one  of  a  pair 
of  cells  which  happen  to  be  more  or 
.  43.  _  Conjugation  in  less  opposite  to  each  other  (Fig.  44, «.). 

Spirogyra,  showing  in  one  This  is  shortly  followed  by  the  forma- 

Filament    solitary    Cells  , .            .             r    ..                .  J. 

(s.c.)  which  have  failed  tlon   of    a    similar    protuberance    on 

to  mate,  and  in  the  other  the  wall  of  the  other  cell  (b.\      The 

82 

two  protuberances  meet  (c.)  and  fuse 
together,  and  the  cell-walls  at  the 
point  of  union  are  dissolved  away  (d.).  Thus  a  hollow  canal  is 
formed  placing  the  cavities  of  the  two  conjugating  cells  in  free 
communication  with  one  another.  In  the  meantime  changes 
are  going  on  in  the  protoplasmic  contents  of  the  conjugating 
cells,  essentially  similar  in  the  two  members  of  each  pair  but 
with  one  cell  still  taking  the  lead  and  the  other  lagging  somewhat 


s.c. 


(Zygotes    (zyg.),     X 
Prom  a  photograph.) 


CONJUGATION  IN   SPIEOGYEA 


97 


behind.     The  chloroplastid   breaks   up ;   the  primordial   utricle 

retreats  from  the  cell-wall  towards  the  middle,  and  the  entire 

protoplasmic  contents   round   themselves   off    into    a    compact 

nucleated  mass — the  gamete  (gam.).     The  time  has  now  arrived 

for  the  all-important  event ;  the  gamete  from  one  cell-chamber 

creeps  through  the  canal  into  the  other  chamber  and  conjugates 

with  the  gamete  which  there  awaits 

it   (Fig.  44,  e.).     The  result  is  a 

zygote  or  zygospore  (Figs.  43,  44, 

zyg.)  which  surrounds  itself  with  a 

thick    protective    envelope    within 

the*    cell-wall   of    the    parent    and 

after  a  period  of  rest  germinates, 

giving  rise  by  repeated  transverse 

division  to  a  new  filament. 

In  this  case  it  will  be  observed 
that  the  two  conjugating  gametes 
are  morphologically  alike,  and  we 
may  therefore  describe  the  process 
of  conjugation  as  isogamous.  As  in 
the  case  of  Bodo,  however,  there  is 
a  physiological  distinction  between 
the  two  as  regards  their  activity ; 
the  gamete  which  takes  the  lead 
throughout  the  whole  process  and 
finally  crawls  through  the  canal  of 
communication  is  obviously  to  be 
regarded  as  male,  and  its  partner 
as  female.  Moreover,  one  entire 
filament  may  produce  nothing  but 
male  gametes  and  may  therefore 
be  regarded  as  a  male  plant  (Fig. 
44,  <? ),  while  the  other  may  produce 
only  female  gametes  and  therefore  be  regarded  as  a  female  plant 
(Fig.  44,  ?  ).  In  the  closely  related  form,  Zygogonium,  there  is 
no  distinction  of  any  kind  between  the  two  sexes,  both  gametes 
exhibit  the  same  degree  of  activity  and  they  meet  each  other  half- 
way, in  the  connecting  canal  between  their  respective  chambers, 
to  form  the  zygote. 

In  Spirogyra,  as  we  have  just  seen,  every  cell  of  the  adult  plant 
is  a  potential  gamete  or  germ  cell ;  in  the  case  of  Volvox,  it  will 

B.  H 


FIG.  44.  —  Diagram  of  two  con~ 
jugating  filaments  of  Spiro- 


a.  —  e.,  pairs  of  cells  in  successive 
stages  of  conjugation;  £  gam., 
male  gametes;  $  gam.,  female 
gametes;  S.C.,  solitary  cell; 
zyg->  zygote. 


V      A 


98 


OUTLINES   OF  E VOLUTION ABY  BIOLOGY 


be  remembered,  this  is  not  so,  certain  cells  only  being  concerned 
in  the  sexual  process.  We  have  here  a  kind  of  fore-shadowing  of 
that  distinction  into  somatic  or  body  cells  and  sexual  or  germ 
cells  which  is  so  characteristic  of  the  higher  organisms,  both 
plants  and  animals.  We  see  a  somewhat  similar  fore-shadowing 


FIG.  45.- 


-Fucus  vesiculosus,  about  half  nat.  size.     (From  Vines'  "  Botany.") 
&,  air  bladders  ;  /,  fertile  branch. 


even  in  the  unicellular  Paramoecium,  except  that  the  distinction 
is  here  confined  to  the  nuclei,  the  meganucleus  being  concerned 
with  the  life  of  the  individual  and  the  asexual  process  of  ordinary 
cell-division,  while  the  micrcnucleus  is  alone  concerned  in  the  / 
sexual  phenomena.     We  can,  in  this  case,  distinguish  between] 
somatic  nucleus  and  germ  nucleus,  though  not  between  sornatia 
and  germ  cells. 


GERM  CELLS  AND   SOMATIC   CELLS  99 

The  loss  of  the  power  to  act  as  germ  cells  or  gametes  which 
the  vast  majority  of  the  constituent  cells  in  a  typical  multicellular 
organism  usually  suffer  is  undoubtedly  one  of  the  penalties 
which  they  have  to  pay  for  their  high  degree  of  speciali- 
zation. The  germ  cells  themselves,  on  the  other  hand,  always 
remain  in  a  more  primitive,  less  specialized  condition,  and 
may,  in  fact,  be  regarded  as  so  many  unicellular  Protista 
enclosed  within  the  multicellular  body.  They  resemble  the 
free-living  Protozoa  and  Protophyta  also  in  that  they  exhibit  a 
certain  degree  of  independence  and  are  in  the  majority  of  cases 
actually  set  free  from  the  parent  body  as  unicellular  individuals  ; 


FIG.  46. — Fucus  vesiculosus ;  Section  through  a  female  Conceptacle,  X  50. 
(From  Vines'  "  Botany,"  after  Thuret.) 

short-lived,  it  is  true,  unless  they  happen  to  meet  one  another 
and  conjugate,  but  nevertheless  enjoying  that  liberty  which  is 
only  possible  to  independent  organisms. 

All  this  is  very  beautifully  illustrated  in  the  case  of  the  common 
brown  seaweed  or  bladder-wrack,  Fucus  vesiculosus  (Fig.  45). 
The  entire  plant  consists  of  flattened  branches,  copiously 
subdivided  and  attached  by  a  root,  usually  to  some  rock  between 
tide-marks.  Here  and  there  the  branches  are  swollen  out  to  form 
air-bladders  (6),  which  serve  as  organs  of  flotation.  The 
histological  structure  of  the  plant  is  comparatively  simple  and 
need  not  detain  us  ;  we  are  concerned  only  with  its  reproductive 
processes. 

Fucus  vesiculosus  is  dioscious  or  unisexual,  there  being  distinct 

H  2 


100       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

male  and  female  plants.  In  both  cases  certain  branches  (/), 
usually  described  as  fertile,  are  characterized  by  the  presence  of 
numerous  minute  spherical  pits,  opening  on  to  the  surface  by  narrow 
mouths.  These  pits  or  conceptacles,  one  of  which  is  represented 
in  vertical  section  in  Fig.  46,  contain  the  sexual  organs,  male 
antheridia  or  female  oogonia,  as  the  case  may  be,  intermingled 
with  hair-like  structures  known  as  paraphyses.  The  antheridia 
(Fig.  47,  a)  are  attached  to  the  branching  paraphyses  in  the  male 


FIG.  47. — Fucus  vesiculosus. 

a,  branching  paraphysis  from  male  conceptacle,  bearing  antheridia ;  &,  an  oogonium 
surrounded  by  unbranched  paraphyses  and  with  its  contents  divided  into  eight  ova ; 
c,  a  discharged  ovum  surrounded  by  spermatozoa,  one  of  which  will  fertilize  it ;  d,  a 
developing  embryo  ;  all  x  160.  (From  Vines'  "Botany,"  after  Thuret.j 

cptfe'eptacles  in  the  form  of  small  sacs  in  which  the  male  gametes 
/(spermatozoa)  are  produced.  These  are  minute,  nucleated,  pear- 
shaped  cells,  each  with  two  flagella ;  they  are  produced  in  large 
numbers  in  each  antheridium  and  set  free  by  rupture  of  the  wall 
of  the  latter  to  make  their  way  out  of  the  opening  of  the 
conceptacle  by  their  own  activity. 

The^oogonia,  are  oval  sacs,  a  good  deal  larger  than  the  antheridia, 

and  occur  amongst  the  hair-like  paraphyses  in  the  female  ,,0011-, 

,_c£p£acle>.(Fig.  46).      In  each  oogonium  (Fig.  47,  b)  eight  female 

gametes  (egg  cells,  ova  or  oospheres)  are  formed  by  cell-division. 

These  are  spherical,  nucleated  cells,  very  much  larger  than  the 


ALTERNATION  OF  GENERATIONS  IN'PJjATJTS  X&l 

spermatozoa,  owing  to  the  great  amount  of  cytoplasm  which  they 
contain.  They  are  liberated  by  rupture  of  the  oogonium  and 
discharged  through  the  opening  of  the  conceptacle  on  to  the 
surface  of  the  plant.  There  they  are  found  by  the  spermatozoa, 
which  swarm  around  them  in  large  numbers,  endeavouring  to 
conjugate  (Fig.  47,  c).  Finally  a  single  spermatozoon  succeeds 
in  boring  its  way  into  each  large  egg  cell,  and  fertilization  is 
effected  by  the  union  of  the  male  and  female  nuclei.  The  zygote, 
well  supplied  with  food  material  by  the  egg  cell,  begins  to  undergo 
cell-division  immediately,  forming  a  rnulticellular  embryo 
(Fig.  47,  d)  which  attaches  itself  by  roots  and  grows  into  a  plant 
resembling  the  parents. 

Here  we  have  a  perfectly  typical  case  of  differentiation  of  the 
gametes  or  germ  cells  into  large  passive  female  ova  and  small 
active  male  spermatozoa,  and  conjugation  is  anisogamous,  the 
ovum  being  "  fertilized  "  by  the  spermatozoon.^ 
f  In  the  ferns,  mosses  and  other  more  highly  organized  plants 
a  new  complication  is  introduced  by  the  fact  that  two  distinct 
forms  of  the  plant  alternate  with  one  another  in  the  life  cycle. 
In  one  only  of  these  forms,  known  accordingly  as  the  garnetophyte, 
does  a  sexual  process  occur ;  the  otirier,  known  as  the  sporophyte, 
reproduces  by  means  of  unicellular  spores,  which  are  produced 
asexually  and  develop  into  new  individuals  without  any  process 
of  conjugation.  The  gametes  or  germ  cells,  borne  on  the  garneto- 
phyte, on  the  other  hand,  conjugate,  and  the  zygote  develops, 
not  into  another  garnetophyte  but  into  a  sporophyte,  while, 
conversely,  the  spores  produced  by  the  sporophyte  develop  into 
gametophytesf~[ 

This  alternation  of  sexual  and  asexual  generations  is  a 
phenomenon  of  very  wide-spread  occurrence  in  the  vegetable 
kingdom,  and,  as  we  shall  see  in  our  next  chapter,  something  of 
the  same  kind  occurs  also  in  certain  rnulticellular  animals. 

Take,  for  example,  any  ordinary  fern.  The  conspicuous  plant 
(Fig.  48)  is  the  sporophyte.  It  is  very  highly  organized  and  shows 
the  typical  differentiation  into  root,  stem  and  leaf  met  with  in  all 
the  higher  groups  of  the  vegetable  kingdom.  Some  or  all  of  the 
leaves  sooner  or  later  produce  on  their  lower  surfaces  sporangia 
(Fig.  48,  A,  C),  little  sac-shaped  structures  in  which  the  spores 
arise  by  division  of  mother  cells  into  fours.  These  spores  are 
liberated,  by  rupture  of  the  sporangia,  in  the  form  of  fine  brown 
dust,  which  may  be  carried  to  considerable  distances  by  the  wind- 


11X2      -OUTLINES  OF  EVOLUTIONARY  BIOLOGY     » 


FIG.  48. — The  Sporophyte  Generation  of  a  Fern,  Aspidiumfilix  mas.    (From 

Strasburger.) 

A,  section  through  a  sorus  or  group  of  sporangia,  covered  by  the  iiidusium  ( x  20,  after 
Kny) ;  B,  lower  surface  of  a  pinna,  showing  the  indusia  covering  the  sori ;  C,  lower 
surface  of  a  pinna  with  the  sori  exposed. 


LIFE  HISTOEY  OF   A  FERN 


103 


PIG.  49. — Diagram  of  a  young  Prothallus 
(pth.)  formed  by  Germination  of  a 
Fern  Spore. 

rh.,  rhizoid  or  root  hair;  sp.c.,  spore  coat. 


If  one  of  them  alights  on  a  suitable  spot,  in  a  moist  and  shady 
situation,  it  may  germinate  (Fig.  49).  Its  thick  outer  wall  rup- 
tures and  a  delicate  tube  is 
put  forth,  containing  the  pro- 
toplasm  and  nucleus.  Cell- 
division  takes  place  and  results 
presently  in  the  formation  of 
the  gametophyte. 

The  gametophyte  of  the 
fern  (Fig.  50)  is  known  as  a 
prothallus.  It  is  an  indepen- 
dent, self-supporting  plant, 
but  much  less  highly  orga- 
nized than  the  sporophyte,  consisting  usually  of  a  green,  heart- 
shaped  plate  of  cells,  not  more  than  perhaps  a  quarter  of  an 
inch  in  diameter,  and  attached  to  the  substratum  by  delicate 
hair-like  rhizcids.  It  develops  no  vascular  system  but  never- 
theless obtains  its  food 
in  the  same  way  as  the 
sporophyte,  absorbing 
water  containing  dis- 
solved mineral  salts 
from  the  soil  by  means 
of  its  rhizoids,  and 
splitting  up  carbon 
dioxide,  obtained  from 
the  air,  by  aid  of  its 
chlorophyll.  Suchpro- 
thalli  are  frequently 
to  be  found  attached  to 
the  surfaces  of  flower- 
pots and  walls  in  damp 
greenhouses  and  other 
places  where  ferns  are 
grown. 

The  sexual  organs, 
male  antheridia  (Fig. 
50,  an)  and  female  archegonia  (Fig.  50,  ar),  are,  like  the  rhizoids, 
found  on  the  lower  surface  of  the  prothallus,  both  usually  occurring 
on  one  and  the  same  plant,  which  is  therefore  monoecious  or 
hermaphrodite.  The  antheridia  (Fig.  51,  a)  are  essentially  similar 


FIG.  50. — The  Gametophyte  Generation  or  Pro- 
thallus of  a  Fern,  Aspidium  filix  mas,  X  8. 
(From  Strasburger.) 

A,  lower  surface  of  a  sexually  mature  prothallus,  showing 

antheridia  (an),  ai*chegonia  (ar),  and  rhizoids  (rh). 

B,  an  older  prothallus  with  the  young  sporophyte  genera- 

tion or  fern  plant  (&,  iv)  attached  to  it. 


104        OUTLINES   OF   EVOLUTIONAEY  BIOLOGY 


to  those  of  Fucus,  being  hollow  sacs  in  which  the  male  gametes 
are  developed.     The  latter  (Fig.  51,  s.)  .are  active  spermatozoa, 

each  having  a  spirally  coiled  body, 
consisting  chiefly  of  chromatin 
material,  and  bearing  a  bunch  of 
cilia  at  one  end,  by  the  vibration  of 
which  the  gamete  swims  actively 
about  in  any  dew  or  other  moisture 
which  may  be  deposited  on  the 
prothallus. 

The  archegonia  (Fig.  52)  differ 
considerably  from  the  oogonia  of 
Fucus,  having  a  characteristic 
structure  which  is  more  or  less 
accurately  repeated  in  the  corre- 
sponding organs  of  all  the  higher 
plants.  Each  consists  of  a  hollow 
swollen  venter,  sunk  in  the  tissue 
of  the  prothallus,  and  a  long  neck 
which  projects  from  the  surface, 
and  the  wall  of  which  is  composed 
of  four  rows  of  cells.  The  venter 
contains  a  single  relatively  large  ovum  or  oosphere  (Figs.  52,  A,  o, 
and  52,  B),  above  which  an  axial  row  of  canal  cells  (Kr,  K") 


?IG.  51. — Antheridiumof  aFern, 
discharging  Spermatozoa 
(Antherozoids)  from  its 

(From  Vines'  "  Botany.") 
a,  antheridium ;  s,  spermatozoon. 


I'IG.    52. — Archegonia  of  a    Fern,   Polypodiam    vulgare,    X    240.      (From 

Strasburger.) 

A,  young,  showing  ovum  (o)  and  canal  cells  (K'^K"),  and  with  the  end  of  the  neck  closed. 

B,  mature,  with  the  end  of  the  neck  open. 

extends  into  the  neck.     When  the  ovum  is  ready  for  fertilization 
the  canal  cells  degenerate  into  mucilage  and  the  cells  at  the  end 


CONJUGATION   IN  FEKNS  105 

of  the  neck  separate  so  as  to  form  an  opening  (Fig.  52,  B).  The 
spermatozoa  appear  to  be  attracted  to  the  opening  by  an  acid 
secretion  discharged  therefrom.  One  of  them  makes  its  way 
down  the  neck  to  the  ovum  and  fertilizes  it  by  the  usual  process 
of  conjugation. 

The  zygote  begins  to  develop,  by  cell-division,  within  the 
venter  of  the  archegonium,and  forms  a  young  sporophyte,  which 
for  some  time  remains  attached  to  the  prothallus  as  shown  in 
Fig.  50,  B,  drawing  nutriment  therefrom  by  means  of  a  special 
temporary  organ  known  as  the  foot.  Presently,  root,  stem  and 
leaf  are  developed  and  the  sporophyte  becomes  self-supporting. 

One  very  remarkable  fact  in  connection  with  the  sexual 
process  in  the  fern  remains  to  be  noticed.  The  gametes,  as  we 
have  seen,  are  normally  produced  in  special  sexual  organs  and 
are  themselves  perhaps  as  highly  differentiated  in  relation  to 
the  function  of  conjugation  as  gametes  ever  are.  It  has  been 
found,  however,  that  if  the  normal  sexual  union  between  ova 
and  spermatozoa  be  prevented  a  conjugation  may  take  place 
between  nuclei  from  adjacent  vegetative  cells  of  the  prothallus, 
resulting  in  the  formation  of  an  embryo  sporophyte  by  so-called 
apogamy.  In  these  cases  it  is  obvious  that  the  sexual  process 
is  not  really  suppressed,  but  simply  transferred  to  ordinary 
prothallial  cells,  which,  though  they  do  not  conjugate  under 
normal  circumstances,  have  retained  the  power  of  so  doing  when 
occasion  arises.  It  seems  probable,  however,  that  in  some  cases 
true  apogamy,  or  suppression  of  the  sexual  process,  occurs,  the 
embryo  sporophyte  arising  from  the  prothallus  without  any 
conjugation  of  gametes, 

The  gametophyte  or  prothallus  of  an  ordinary  fern  is,  as  we 
have  already  seen,  produced  by  the  development,  of  a  unicellular 
spore  (Fig.  49),  and  in  most  cases  is  monoecious  or  hermaphro- 
dite, bearing  both  male  and  female  sexual  organs  and  male  and 
female  gametes.  In  the  less  commonly  known  and  not  very 
fern-like  "  heterosporous  "  forms,  however  (Isoetes,  Salvinia  and 
Marsilea),  the  gametophyte  is  dioecious  or  unisexual,  there  being 
distinct  male  and  female  prothalli,  and  this  sexual  differentia- 
tion affects  not  only  the  prothalli  themselves  but  the  spores  from 
which  these  are  developed.  Hence  in  these  forms  we  find  small 
microspores  which  produce  male  prothalli,  and  large  megaspores 
which  produce  female  prothalli.  The  spores  themselves  are  set 
free  from  the  parent  sporophyte,  but  the  prothalli  are  very  much 


106        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

reduced  in  size  and  never  become  free  from  the  spores ;  they 
nevertheless  develop  antheridia  and  archegonia  respectively, 
in  which  spermatozoa  and  ova  are  produced,  and  from  the 
conjugation  of  these  arise  zygotes  or  fertilized  ova  which 
develop  into  new  sporophytes. 

We  have  briefly  noticed  these  heterosporous  ferns  because, 
as  regards  the  sexual  phenomena  which  they  exhibit,  they 
constitute  a  very  interesting  connecting  link  between  the  ordinary 
(homosporous)  ferns,  which  produce  only  one  kind  of  spore,  and  the 
highest  members  of  the  vegetable  series,  «he  flowering  plants. 
<^71n  the  flowering  plants  an  alternation  of  sexual  and  asexual 
generations  can  still  be  traced,  but  here  the  gametophyte  is  so 
much  reduced  in  size  and  has  become  so  degenerate  in  structure 
that  it  is  quite  inconspicuous,  and  can  only  be  detected  by  micro- 
scopical examination  and  recognized  as  constituting  a  distinct 
generation  in  the  light  of  our  knowledge  of  lower  forms. 

The  flowering  plant  itself  is  the  sporophyte,  and  it  is  hetero- 
sporous, producing  microspores  and  megaspores.  The  pollen 
grains  are  the  microspores,  while  the  megaspores  are  represented 
by  the  embryo  sacs  enclosed  within  the  ovules  or  unripe 
seeds.  The  microspores,  like  the  spores  of  ferns,  are  set  free 
from  the  parent  sporophyte,  the  megaspores,  however,  are  never 
set  free  as  such,  and  in  neither  case  does  the  gametophyte  become 
ree  from  the  spore. 

The  terms    pollen   grain   and   embryo   sac    were   applied  to 

e  structures  in  question  long  before  their  true  nature  as 
microspores  and  megaspores  was  recognized,  and  they  have 
become  so  firmly  established  that  it  is  hardly  possible  to  avoid 
using  them. 

If  we  examine  any  typical,  fully  developed  flower,  such  as  is 
represented  diagrammatically  in  Fig.  53,  we  shall  find  that  it 
consists  of  four  whorls  or  circlets  of  specially  modified  leaves. 
Beginning  at  the  outside  we  find  first  the  calyx  (Ke),  composed 
of  a  number  of  sepals,  which  usually,  but  by  no  means  always, 
retain  the  green  colour  characteristic  of  leaves  and  serve  mainly  for 
the  protection  of  the  inner  parts  of  the  flower  while  in  the  bud  ; 
then  the  corolla  (K),  composed  of  petals,  which  may  be  brightly 
coloured  and  serve  to  attract  insects ;  then  the  androecium 
composed  of  stamens  (a,f);  and  lastly,  in  the  centre  of  the 
flower,  the  gynoecium  or  pistil  (n,  g,  F),  composed  of  carpels. 
The  stamens  and  carpels  are  often  spoken  of  as  the  essential 


STRUCTURE   OF  A   FLOWER 


107 


parts  of  the  flower.     They  are  really  to  be  regarded  as  spore- 
bearing  leaves  or  sporophylls. 

Each  stamen  consists  usually  of  a  long  stalk  or  filament  (/), 
bearing  an  anther  (a)  at  its  extremity.  The  anther  is  a  bilobed 
structure,  and  each  lobe  contains  two  chambers,  or  pollen  sacs, 
in  which  the  pollen  grains  (p)  are  formed  by  the  division  of 
mother  cells  into  fours,  just  as  the  spores  of  an  ordinary  fern 
are  developed  within  the  spo- 
rangia. The  pollen  sacs  are,  in 
[act,  nothing  but  sporangia — 
and  microsporangia,  because  they 
contain  microspores. 

The  pistil  is  formed  of  a  vary- 
ing number  of  carpels,  which, 
dther  singly  or  united,  give  rise 
to  a  closed  chamber  below,  the 
so-called  ovary1  (F),  surmounted 
by  a  longer  or  shorter  style  (g), 
ending  above  in  a  rounded  viscid 
surface,  the  stigma  (?i).  In  the 
interior  of  the  ovary,  attached 
to  the  carpels,  are  developed  the  FIG.  53.— Diagram  of  a  typical 
ovules  (S),  which  are  nothing  Mower  in  vertical  Section. 

,  (Prom  Vines'  "  Botany.") 

but    sporangia    (megasporangia)    a,  an;her;  em>  embryo  sac; 

enclosed  each  in  a  double  enve- 
lope (i).  In  each  ovule  a  single 
embryo  sac  or  megaspore  (em) 
is  produced.  Only  one  ovule 
is  represented  in  the  diagram,  but  there  are  usually  a  large 
number  in  each  ovary. 

Having  thus  briefly  described  the  parts  of  the  sporophyte 
with  which  we  are  immediately  concerned,  we  must  turn  our 
attention  for  a  few  moments  to  the  gametophyte.  The  male 
gametophyte,  which  never  consists  of  more  than  a  very  small 
number  of  cells,  is  developed  from  the  pollen  grain  or  microspore. 
This  latter  is  at  first  a  perfectly  typical  unicellular  spore, 
with  a  single  nucleus  surrounded  by  cytoplasm,  and  the  whole 
enclosed  in  a  thick  protective  cell-wall  often  ornamented  with 
microscopic  sculpture  of  various  patterns.  The  commencement 

1  This  is  a  very  unfortunate  name  because  the  structure  in  question  is  an  entirely 
different  kind  of  organ  from  the  ovary  of  animals. 


f,  filament  of  stamen;  F,  wall  of 
ovary ;  g,  style ;  i,  integument  of 
ovule;  K,  corolla;  Ke,  calyx;  n, 
stigma ;  p,  pollen  grains ;  ps,  pollen 
tube ;  S,  ovule. 


108        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


of  germination  of  this  spore  in  typical  cases  (Fig.  54)  is 
marked  by  the  division  of  the  nucleus  into  two.  Around  one  of 
these  two  cytoplasm  collects  to  form  a  naked  "antheridial  cell" 
(m)  ;  the  other,  with  the  remainder  of  the  cytoplasm,  constitutes 
the  "  vegetative  cell  "  (k),  which  may  or  may  not  divide  again. 
The  antheridial  cell  divides  into  two  "  generative  cells."  The 
vegetative  cell,  or  cells,  represents  the  last  vestige  of  the 
body  of  the  male  prothallus  ;  the  generative  cells  are  male  gametes. 
The  germination  of  the  pollen  grain  and  development  of  the 

male  prothallus  are  completed  by 
the  putting  forth  of  the  pollen  tube 
(Fig.  53,  ps,  Fig.  54),  -which  takes 
place  if  the  pollen  grain  is  fortunate 
enough  to  alight  upon  the  stigma  of 
a  flower  of  the  right  kind.  The 
pollen  tube  forces  its  way  through 
the  loose  tissue  of  the  style  to  the 
ovary  and  comes  into  intimate  rela- 
tions with  one  of  the  ovules  contained 
therein. 

The  female  gametophyte  is  repre- 
sented   by   a  few    cells    formed    by 

.      .  of  the  division   of,  the  ^egaspore  (embryo 

Pollen   Grain   of  Lilium  sac),  or  rather  of  its  nucleus,  and  to 

martagan,  X  375.  (From  some  extent  Of  its  cytoplasm,  within 

Strasburger,  after  ,,              ,         mi 

Guignard.)  *ne  ovule.     The  process  is  a  some- 

*,  nucleus  of  the  vegetative  ceil  of    what  complicated  one,  but,  without 

^y^^sfsS^  g°ins  int°  details>  we  ma?  note  that 

by  division  of  the  antheridial  at  the  time  when  the  ovule  is  ready 

for  "  fertilization  "  the  embryo  sac  in  a 

typical  flowering  plant  (Angiosperm)  contains  seven  cells,  one  of 
which  is  a  female  gamete  (ovum  or  oosphere),  while  the  others 
may  be  taken  to  represent  the  female  prothallus,  including  a 
vestige  of  an  archegonium.  The  arrangement  of  these  cells  is 
shown  in  Fig.  55  (at,  e,  k,  s). 

The  embryo  sac  or  megaspore  (E)  is  surrounded  by  a  cellular 
layer  known  as  the  nucellus  (Fig.  55,  K),  which  represents  the  wall 
of  the  sporangium,  and  this  in  turn  by  two  other  coats  (ai  and  ii), 
the  outer  and  inner  integuments  of  the  ovule,  which  grow  up 
around  the  nucellus.  The  entire  ovule  is  attached  to  the  wall 
of  the  ovary  by  a  stalk  or  funiculus  (/),  upon  which  it  is 


FIG. 


CONJUGATION   IN  FLOWERING  PLANTS 


109 


frequently  bent  sharply  round  as  shown  in  Fig.  55.  Opposite 
to  the  spot  where  the  funiculus  is  attached  to  the  ovule  an 
aperture  is  left  in  the  integuments  known  as  the  micropyle  (m). 
It  is  through  this  micropyle  that  the  tip  of  the  pollen  tube 
usually  forces  its  way  in  search  of  the  female  gamete  (e),  which 
lies  at  the  end  of  the  embryo  sac  close  to  it.  The  wall  of  the 
pollen  tube  and  that  of  the  embryo  sac  are  absorbed  where 
they  come  in  contact  with  one 
another,  and  thus  a  passage  is 
opened  for  the  male  gamete, 
which  passes  down  the  pollen 
tube  into  the  embryo  sac  and 
there  conjugates  with  the  ovum. 
The  zygote  thus  formed  develops 
within  the  embryo  sac  into  an 
embryo  sporophyte1;  the  integu- 
ments of  the  ovule  become  har- 
dened to  form  the  seed  coat; 
reserve  food  material,  such  as 
starch  or  oil,  is  stored  up 
either  in  the  embryo  itself  or 
in  th  s  endosperm2  around  it,  and 
the  ovule  and  its  contents 
separate  from  the  parent  sporo- 
phyte as  the  ripe  seed  (Fig. 
56,  A). 

When  the  seed  ripens  the 
development  of  the  contained 
embryo  is  suspended  for  an 
indefinite  period,  to  be  resumed 
again  if  and  when  the  seed  finds 
a  suitable  situation  in  which  to  germinate.  The  embryo  may  then 
continue  its  development  into  the  adult  sporophyte  (Fig.  56,  B). 

It  is  interesting  to  notice  that  no  less  than  three  distinct 
generations  take  part  in  the  formation  of  the  seed.  The  seed 
coat  belongs  to  the  parent  sporophyte,  while  the  contents  of  the 

1  Vide  pp.  48—50  (Fig.  14). 

2  The  second  generative  nucleus  from  the  pollen  tube  conjugates  with  the  nucleus 
in  the  middle  of  the  embryo  sac  (Fig.  55  &),  and  the  "  endosperm  "  arises  by  repeated 
division   of  the  nucleus  resulting  from  this  conjugation.      The   whole  process  is 
somewhat  complex  and  it  is  not  essential  to  our  present  purpose  to  describe  it  in 
detail. 


FIG.  55. — Diagram  of  the  unfer- 
tilized Ovule  of  a  Flowering 
Plant  (Angiosperm)  in  longi- 
tudinal Section.  (From  Vines' 
"Botany.") 

ai,  outer  integument ;  at,  antipodal  cells  ; 
<?,ovum ;  E,  embryo  sac  ;  /",  funiculus  ; 
ii,  inner  integument ;  Jc,  central  or 
definitive  nucleus  of  embryo  sac ;  Kt 
nucellus  ;  m,  micropyle  ;  s,  synergidee. 


110       OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


tc^t. 


cot. 


rad,. 


embryo  sac  at  first  represent  the  female  gametophyte,  which  dis- 
appears as  the  zygote  develops  into  the  embryo  of  another 
sporophyte.  In  this  way  a  very  intimate  relation  is  established 
between  each  sporophyte  generation  and  the  one  which  precedes 
it,  and  the  gametophyte  is  crushed  out  of  existence,  as  an  inde- 
pendent generation,  between  the  two. 

Accompanying  this  almost  total  suppression  of  the  gametophyte 
we  find  a  delegation  of  certain  responsibilities  connected  with  the 
sexual  function  to  the  asexual  sporophyte,  and  the  development 
by  the  latter  of  what  may  be  termed  vicarious  sexual  characters. 

These  characters  find 
their  expression  in  •  that 
most  remarkable  feature 
of  all  the  flowering 
plants,  the  flower  itself. 
Thus  the  use  of  the 
terms  male  and  female 
may  be  extended  in  the 
first  instance  to  the  sta- 
mens and  pistil,  though 
these  are  really  merely 
the  spore-bearing  leaves 
of  the  asexual  genera- 
tion. Similarly  the 

transference  of  the  pol- 
FIG.  56.-Garden  Pea  (Pisum  sativum}.          len  graing  from  gtamens 

A,  ripe  seed  split  in  half ;  B,  seed  germinating.  ,        ,.                    £,               , 

cot.,  cotyledons  or  seed  leaves  -,  pi.,  plumule  or  shoot  of  to  Stigma  IS  Often  Spoken 

embryo;  rad.,  radicle  or  root  of  embryo;  test.,  Qf   ag  ^e  fertilization  of 

testa  or  seed  coat. 

the  flower,  though  it  is 

obviously  not  the  true  process  of  fertilization  but  only  a  necessary 
preliminary  to  the  conjugation  of  the  gametes,  and  is  therefore 
more  accurately  spoken  of  as  pollination. 

It  will  have  been  observed  from  the  foregoing  description  that  I 
not  only  is  the  male  gametophyte  of  the  flowering  plant  reduced  j 
almost  to  the  point  of  disappearance,  but  the  male  gamete  itself  f 
has  apparently  suffered  great  degeneration.     It  is  no  longer  an 
active  spermatozoon,  swimming  about  by  means  of  flagella  or 
cilia,  as  in  Eudorina  or  in  the  ferns,  but  a  shapeless  nucleated 
mass  of  protoplasm  which  has  at  the  most  a  sort  of  amoeboid 
power  of  locomotion.      By  far  the  greater  part  of  the  travelling 
which  it   has  to   accomplish   in   order  to   reach   the   ovum   is 


ra-d. 


POLLINATION  111 

effected  while  it  is  still  enclosed  within  the  pollen  grain  and  at 
the  expense  of  some  external  agency,  for  it  is  not  until  the  pollen 
grain  has  alighted  upon  the  stigma  and  the  pollen  tube  is  put 
forth  that  the  "  generative  cell "  begins  to  exercise  its  cwn  feeble 
powers  of  locomotion. 

The  transference  of  the  pollen  to  the  stigma  is  effected  usually 
in  one  of  two  ways,  either  by  the  action  of  the  wind  or  by  the 
agency  of  insects.  Flowers  which  are  pollinated  in  the  first  of 
these  two  ways  are  said  to  be  anemophilous,  and  they  are  usually 
small  and  inconspicuous,  as  in  the  grasses  and  plantains,  and  many 
forest  trees.  It  is  a  very  extravagant  method  of  pollination, 
involving  the  production  of  enormous  quantities  of  pollen,  most 
of  which  is  wasted,  for  only  a  very  minute  percentage  of  the 
pollen  grains  will  ever  chance  to  alight  upon  stigmas.  We 
realize  this  when  we  see  the  enormous  quantities  of  yellow 
pollen  dust  which  are  blown  off  the  pine  trees  in  spring  time. 
The  entomophilous  or  insect-pollinated  flowers,  on  the  other  hand, 
have  hit  upon  a  much  more  economical  way  of  doing  the 
business.  The  development  of  nectar  or  honey  as  a  bait  for 
their  insect  visitors,  and  of  gaily  coloured  petals  or  sepals  and 
sweet  scents  as  a  means  of  attracting  them,  and  in  many  cases 
of  elaborate  mechanical  contrivances  to  secure  that  the  insect 
shall  not  obtain  its  reward  without  doing  the  work  of  pollination, 
all  co-operate  in  bringing  about  the  desired  end,  and  the  study 
of  these  various  adaptations  forms  one  of  the  most  interesting 
chapters  in  biological  science.  We  shall  refer  to  it  again  when 
we  come  to  deal  with  adaptation  and  natural  selection. 

In  many  cases  a  complete  sexual  differentiation  is  manifested 
by  the  entire  flowers  themselves,  some  having  stamens  without 
carpels  and  others  carpels  without  stamens,  and  being  spoken  of 
as  "male"  or  "female"  flowers  accordingly.  We  see  this,  for 
example,  in  the  case  of  the  vegetable  marrow,  where  both  kinds 
of  flower  are  borne  on  the  same  plant,  while  in  other  cases,  such 
as  the  weeping  willow  and  the  Japanese  Aucuba,  the  whole  plant 
may  be  either  "  male  "  or  "  female,"  producing  flowers  of  the  one 
kind  only.  Thus  the  terminology  which  strictly  speaking  is 
applicable  only  to  the  sexual  gametophyte,  has,  as  a  matter  of 
convenience,  been  extended  to  the  asexual  sporophyte  in  order  to 
describe  the  secondary  sexual  characters  which  have  been  trans- 
ferred to  it  in  consequence  of  the  suppression  of  the  former. 
The  telescoping  of  successive  generations  one  within  the  other 


112        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

— the  embryo  sporophyte  into  the  ovule  of  the  preceding  sporo- 
phyte  generation,  with  the  gametophyte  crushed  in  between — 
is  the  most  characteristic  feature  of  the  life  cycle  of  the  higher 
plants. 

From  the  homosporous  ferns  upwards  throughout  the  vegetable 
series  it  is  obviWs  that  the  gametophyte  is  not  nearly  so  well 
adapted  in  its  organization  to  the  conditions  under  which  the 
higher  plants  have  to  live  as  is  the  sporophyte.  There  has 
apparently  been  a  kind  of  rivalry  between  the  two  alternating 
generations,  in  which  the  gametophyte  has  had  much  the  worst 
of  it.  The  sexual  function  itself,  however,  is  far  too  important 
to  be  altogether  abandoned,  and  so  the  successful  sporophyte  has 
finally  taken  over  many  of  the  responsibilities  connected  there- 
with, while  the  poverty-stricken  gametophyte  has  ultimately 
become  entirely  parasitic  upon  its  rival. 


CHAPTER  IX 

Sexual  phenomena  in  multicellular  animals — Structure  and  life  history  of 
Hydra  and  Obelia — Alternation  of  generations — The  coelomate  type  of 
structure — Secondary  sexual  characters — The  evolution  of  sex. 

fr 

IN  multicellular  animals  or  Metazoa,  as  in  multicellular  plarrts,/- 

a  sharp  distinction  can  usually  be  drawn  between  the  somatic  cells 
which  build  up  the  various  tissues  and  are  concerned  with  the 
life  of  the  individual,  and  the  gerrjQL  cells  or  gametes  which  are 
concerned  jwith  the  propagation  of  the  race  and  which  alone  (in 
most  cases)  have~the  power  of  separating^rom  the  parent  soma 
or  body  and  giving  rise  to  new  individuals. 

In  nearly  all  the  Metazoa_the_gametes  are_sexujjj;y_diffej<en- 
tiated  into  relatively  large,  passive  ova  and  much  more  minute, 
active  spermatozoa  which  swim  about  by  means  of  flagella. 
The  actual  gametes  arise  by  subdivision  of  undifferentiated 
primordial  germ  cells.  In  the  sponges,  whose  organization  has 
not  advanced  very  much  beyond  that  of  complex  colonies  of 
Protozoa,  the  primordial  germ  cells  are  merely  wandering 
amoeboid  cells,  resembling  the  white  blood  corpuscles  of  verte- 
brates. Some  of  these  round  themselves  off  and  give  rise  to 
more  or  less  spherical  ova,  others  divide  into  spermatozoa,  and 
probably  the  entire  sponge  itself  is  in  most  cases  either  male  or 
female,  producing  one  kind  of  gamete  only.  In  the  sponge  the 
germ  cells  are  not  localized  in  definite  organs  but  scattered 
singly  or  in  groups  throughout  the  gelatinous  ground-substance 
of  which  the  body  is  largely  composed. 

In  the  great  majority  of  Metazoa,  on  the  other  hand,  the\ 
germ  cells  are  segregated  in  well-defined  organs  termed  gonads. 
As  a  rule  each  gonad  produces  only  ova,  when  it  is  known  as  an 
ovary ;  or  spermatozoa,  when  it  is  known  as  a  spermary  or  testis ; 
only  occasionally  does  it  produce  both,  as  in  the  case  of  the  ovo- 
testis  of  the  snail.  The  gonads  may  accordingly  be  spoken  of  as 
female,  male,  or  hermaphrodite  as  the  case  may  be,  and  the  same 
terms  are  also  applied  to  the  animals  themselves,  a  male  or  a 


114       OUTLINES   OF   EVOLUTIONAKY  BIOLOGY 

female  animal  possessing  either  male  or  female  gonads,  while  a 
hermaphrodite  animal  may  either  possess  a  combined  ovo-testis 
or  both  ovaries  and  testes  separately. 

In  illustration  of  these  points  we  may  briefly   describe   the 
structure  and  life   history   of  the  common  fresh  water  polype, 


FIG.  57. — The  fresh  water  Polype  (Hydra)  cut  in  half  longitudinally  and 
greatly  enlarged.     (From  Marshall  and  Hurst's  "  Practical  Zoology.") 

A,  mouth  ;  B,  hypostome  ;  C,  digestive  cavity ;  D,  ectoderm ;  E,  mesoglcea ;  F,  endoderm ; 
G,  tentacle ;  H,  testis ;  I,  ovum  in  ovary ;  K,  bud ;  L,  foot. 

Hydra  (Fig.  57),  so  frequently  found  attached  to  aquatic  plants 
in  ponds  and  ditches.  Hydra  is  a  member  of  the  great  group 
Coelenterata,  which  includes  the  sea-firs,  jelly-fish,  sea-anemones 
and  corals,  and  which  are  distinguished  by  the  fact  that  they 
retain  throughout  life  the  fundamental  features  of  the  gastrula 
(compare  Fig.  13,  IX,  X).  The  body  of  a  typical  Ccelenterate 
animal  consists  essentially  of  a  simple  sac  (Fig.  57)  whose  wall 


HYDEA  115 

is  composed  of  two  layers  of  cells.  The  cavity  of  the  sac  (C)  is 
the  digestive  or  gastral  cavity  (enteron),  and  it  has  only  a  single 
opening  to  the  exterior,  the  mouth  (A),  usually  surrounded  by  a 
ring  of  tentacles  (G).  The  wall  of  the  sac  is  solid  and  there  is 
no  body  cavity  or  cceloni  surrounding  the  digestive  tube  as  in 
higher  animals  (Coelomata).  The  outer  cell  layer  (D)  of  the  body 
wall  is  the  ectoderm  (epiblast  of  the  embryo),  the  inner  (F)  is  the 


N, 


PIG.  58. — A  small  portion  of  a  thin  longitudinal  Section  through  the  Body 
Wall  of  Hydra  viridis,  X  800.  (From  Marshall  and  Hurst's  "  Practical 
Zoology.") 

A,  a  large  ectoderm  cell ;  B,  its  nucleus ;  C,  its  muscle  process ;  D,  an  undischarged 
thread  cell ;  E,  its  trigger  process ;  F,  a  thread  cell  with  discharged  thread  ;  G,  inter- 
stitial cells  ;  H,  mesoglcea ;  I,  endoderm  cell ;  K,  vacuole ;  L,  nucleus  of  endoderm 
cell ;  M,  green  algal  cells  living  in  the  endoderm  cells ;  N,  flagellum  of  endoderm  cell. 

endoderm  (hypoblast  of  the  embryo)  and  between  the  two  is  a 
gelatinous,  non-cellular  supporting  lamella,  the  mesogloea  (E). 

Hydra  itself  is  a  very  small  form,  but  easily  recognizable  by 
the  naked  eye.  The  body  is  long  and  slender  or  short  and  thick, 
according  to  its  state  of  contraction,  and  the  same  is  true  of  the 
tentacles,  which  may  be  visible  as  mere  knobs  around  the  mouth 
at  the  unattached  end  of  the  animal,  or  as  long  slender  threads 
extended  through  the  water  like  fishing  lines  and  serving  for  the 
capture  of  the  minute  organisms  upon  which  the  Hydra  feeds. 
The  mouth  is  situated  on  the  top  of  a  conical  projection, 

i  2 


116        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

the  hypostome  (Fig.  57,  B),  which  lies  within  the  circle  of 
tentacles. 

The  endoderm,  which  immediately  lines  the  gastral  cavity,  is 
made  up  of  a  single  layer  of  relatively  large  cells  (Fig.  58,  I) 
whose  function  is  digestive.  The  ectoderm  is  made  up  of  several 
kinds  of  cells,  some  larger  than  others.  The  larger  ones  (A) 
are  much  broader  at  their  outer  than  at  their  inner  ends 
and  the  interstices  thus  left  between  the  "  latter  are  filled  up 
by  small  interstitial  cells  (G).  The  endoderm  cells  and  the 
larger  ectoderm  cells  both  send  out  prolongations  of  their  bodies 
into  the  gelatinous  mesogloea  (H)  which  lies  between  them,  and 
these  processes,  having  the  form  of  elongated  fibres  (C),  are  the 
seat  of  that  power  of  contraction  which  Hydra  possesses  in  such 
ashigh  degree — they  are  in  fact  muscular. 

Hydra  has  two  very  distinct  methods  of  reproduction,  asexual 
and  sexual  respectively.  The  former  consists  in  a  process  of 
budding,  little  hollow  outgrowths  of  the  body  being  formed, 
which  elongate,  acquire  mouth  and  tentacles,  and  for  a  time 
remain  attached  to  the  parent  (Fig.  57,  K).  In  this  way  tem- 
porary colonies  of  polypes  may  be  produced,  but  sooner  or  later 
the  buds  separate  and  begin  to  lead  independent  lives. 

Sexual  reproduction  is  effected  by  means  of  ova  and  sperma- 
tozoa, which  are  essentially  similar  to  those  of  higher  animals. 
They  are  produced  in  gonads — ovaries  and  testes — and  as 
both  kinds  of  ^nad^^aajlv  occur"  in  the  same  individual  the 
animal  is  hej^aphrodite/T'ne  testes  (Fig.  57,  H)  take  the  form 
of  little  swellings  situated  at  a  short  distance  beneath  the  ring 
of  tentacles  and  formed  each  by  an  accumulation  of  interstitial 
ectoderm  cells.  These  are  the  primordial  germ  cells,  by  the 
division  of  which  the  spermatozoa  are  formed.  The  spermatozoon 
is  perfectly  typical,  resembling  a  flagellate  protozoon,  with  an 
ovoid  head  consisting  almost  entirely  of  chromatin  and  a  long 
cytoplasrnic  tail  or  flagellum  by  means  of  which  it  swims 
actively  about  when  shed  into  the  surrounding  water  by  rupture 
of  the  testis. 

There  is  usually  only  a  single  ovary,  appearing  as  a  larger 
projection  from  the  body  wall  nearer  to  the  attached  end  of 
the  animal ;  it  consists  at  first,  like  the  testes,  of  a  heap  of 
primordial  germ  cells  formed  by  the  multiplication  of  interstitial 
cells.  In  each  ovary,  however,  only  a  single  cell  develops  into  a 
mature  ovum  (Fig.  57,  I),  its  sister  cells,  which  may  all  be 


HYDRA 


117 


regarded  as  potential  ova,  being  sacrificed  for  the  benefit  of  the 
one;   in   fact   they  are   simply  devoured   by  the  voracious  egg 


FIG.  59. — Development  of  Hydra.   (From  Bourne's  "  Comparative  Anatomy," 
partly  after  Brauer.) 

A,  the  mature  ovum,  full  of  yolk  granules  and  still  attached  to  the  body  wall  of  the 
parent ;  B,  section  of  blastula  or  blastosphere  produced  by  segmentation  of  the  ovum ; 
C,  the  embryo  becoming  solid  by  migration  into  the  blastocoel  of  cells  cut  off  from 
the  wall  of  the  blastula  to  form  the  hypoblast;  D,  solid  embryo  composed  of  epiblast 
and  hypoblast  and  enclosed  in  a  protective  "shell ;  E,  embryo  flattening  itself  out 
within  the  shell ;  F,  embryo  emerging  from  the  shell  and  with  the  gastral  cavity 
appearing  in  the  endoderm  (hypoblast) ;  Cr,  empty  shell  after  the  escape  of  the 
embryo*^ 

blc,  blastoccel ;  ec,  ectoderm  (epiblast) ;  en,  endoderm  (hypoblast) ;  i,  solid  mass  of  hypo- 
blast  cells ;  mg,  mesoglcea  ;  sh,  shell ;  she,  outer  layer  of  shell ;  shi,  inner  layer  of  shell. 

cell,  which  puts  forth  pseudopodia  and  feeds  upon  them  like  a 
hungry  Amoeba.  In  this  way  the  ovum  attains  a  relatively  large 
size  and  its  cytoplasm  becomes  loaded  with  yolk  corpuscles 


118       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


0. 


which  will  serve  later  on  for  the  nutrition  of  the  developing 
embryo. 

Some   of  the  surrounding   cells    of    the    ectoderm    at    first 

form  a  covering 
or  envelope  for  the 
growing  ovum, 
but  this  is  pre- 
sently ruptured 
and  the  mature 
egg  is  exposed  on 
the  surface  of  the 
body  of  the  parent 
Hydra.  There  i^ 

is  found  by  a  spermatozoon,  which  is 
attracted  towards  it  0,nd  by  its  own 
activity  bores  its  way  into  the  ovum. 
-..This  act  of  fertilization  is  concluded  in 
*ihe  usual  manner  by  the  fusion  of  the 
nucleus  of  the  spermatozoon '(male  pro- 
nucleus)  with  that  of  the  ovum  (female 
pronucleus)  to  form  the  zygote  nucleus. 
The  fertilized  ovum  or  zygote  (Fig. 
59,  A)  undergoes  segmentation  while 
still  remaining  attached  to  the  parent 
Hydra.  In  this  way  a  single-layered, 
hollow  blastula  (B)  is  formed,  which 
becomes  converted  into  a  two-layered 
embryo  by  migration  of  cells  into  the  in- 
terior to  form  the  at  first  solid  hypoblast 
or  endoderm  (C,  D,  ?).  The  epiblast 
or  ectoderm  cells  (D,  ec)  now  secrete  a 
thick  horny  protective  envelope  (sh, 
ski)  around  the  embryo,  which  falls 
off  from  the  parent  and  undergoes  a 
period  of  rest  at  the  bottom  of  the 
pond.  After  a  time  the  interrupted 
development  is  resumed,  the  horny  envelope  is  ruptured,  and 
the  embryo  escapes  (F).  The  gastral  cavity  appears  in  the 
midst  of  the  endoderm  cells,  the  mouth  is  formed  by  perforation 
at  one  end,  and  the  tentacles  bud  out.  With  the  formation  of 
the  mouth  the  gastrula  stage  is  reached,  but  it  will  be  noted  that 


A. 


hyc. 


FIG.  60. — Obelia  geniculata. 
A,  part  of  hydroid  colony;  B, 

free  -  swimming     medusa  ; 

both  x  13.     (From  photo- 


bst 


Dlastostyle ;  gon.,  gonad; 
gth.,  gonotheca ;  hyc.,  hy- 
drocaulus ;  hyd.,  hydranth ; 
hyth.,  hydrotheca ;  mn., 
manubrium ;  p.s.,  perisarc  ; 
r.c.,  radial  canal ;  ten.,  ten- 


OBELTA  119 

this  condition  is  arrived  at  by  a  somewhat  different  route  from 
that  which  leads  to  the  corresponding  stage  in  Amphioxus  (com- 
pared Fig.  13, 1— X). 

Closely  related  to  Hydra  are  a  large  number  of  marine  Goelen- 
terates,  which,  from  their  obviously  animal  nature  combined 
with  their  plant-like  mode  of  growth,  were  known  to  the  older 
naturalists  as  zoophytes.  One  of  the  most  familiar  examples  of 
these  is  Obelia  (Fig.  60),  which  is  frequently  found  attached  to 
rocks  or  seaweeds  near  low  water  mark. 

belia  differs  from  Hydra  in  several  interesting  particulars. 
( In  the  first  place  the  asexual  process  of  multiplication  by  means 
N  of  budding  takes  place  in  a  very  regular  manner,  and  the  buds, 
^instead  of  separating  from  the  parent,  remain  connected  together 
tti  form  permanent  colonies,  (Fig.  60,  A)  in  which  the  constituent 
individuals  or  persons  (sometimes  called  zooids)  are  arranged 
in  a  perJectly  definite  way.     The  colony   is   comparable  to  an 
^ajfeofescent  colony  of  Protozoa  such  as  Zoothamnium  or  Epi- 
stylis  (compare  Fig.  9,  11— 15),  but  the  individuals  of  which  it 
is  composed  are  units  of  a  higher  order  than  single  cells.      — 

In  the  second  place  the  colony  develops  a  common  skeleton, 
secreted  by  the  ectoderm,  which  takes  the  form  of  a  slender 
tube  of  horny  perisarc  (ps.)  enclosing  all  the  branches  and 
expanding  at  the  end  of  each  into  a  little  cup  or  hydrotheca  (hyth.), 
occupied  by  a  single  zooid.  Lastly  the  colony  is  polymorphic, 
the  zooids  exhibiting  a  certain  amount  of  differentiation  and 
division  of  labour  amongst  themselves. 

A  network  of  root-like  branches  at  the  base  of  the  colony 
creeps  over  the  substratum  and  serves  for  attachment.  From 
this  network,  which  is  not  shown  in  the  illustration,  arises  a 
little  forest  of  vertical  stems,  each  of  which  has  a  characteristic 
zig-zag  outline  (Fig.  60,  A).  From  each  angle  of  the  stem  a 
short  branch  is  given  off  which  terminates  in  a  single  hydra -like 
zooid  known  as  a  hydranth  (hyd.),  enclosed  in  one  of  the  horny 
cups  or  hydrothecae,  from  the  mouth  of  which  its  tentacles  are 
extended  into  the  water. 

The  structure  of  the  hydranth  is  similar  in  all  essential 
respects  to  that  of  Hydra.  In  the  middle  of  the  ring  of  tentacles 
is  the  mouth,  situated  on  a  projecting  hypostome  and  leading 
into  the  digestive  cavity,  and  the  different  hydranths  of  the 
colony  are  all  placed  in  communication  with  one  another  by  the 
tubular  hydrocaulus,  or  common  stalk  (hyc.),  enclosed  in  the  horny 


120       OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

perisarc.     These  hydranths  are  the  nutritive  individuals  of  the 
colony,  whose  function  it  is  to  capture  and  digest  the  prey. 

Very  often  we  find,  in  the  angle  between  a  hydranth-bearing 
branch  and  the  main  stem,  a  short  branch  bearing  an  individual 
or  zooid  of  a  different  kind,  enclosed  in  a  horny  cup  of  totally 
different  shape.  These  individuals  are  somewhat  club-shaped ; 
they  have  no  mouth  and  no  -tentacles,  and  they  are  termed 
blastostyles  (Fig.  60,  A,  bst.).  The  horny  cup  in  which  each  is 
enclosed  is  urn-shaped,  with  constricted  mouth,  and  is  distin- 
guished by  the  name  gonotheca  (gth.).  The  function  of  the 
blastostyle  is  exclusively  reproductive  and  it  is  entirely  dependent 
for  its  nutrition  upon  digested  food  received  from  the  hydranths 
through  the  hydrocaulus.  It  reproduces  by  budding,  and  the 
buds  are  often  found  attached  to  it  in  large  numbers,  within  the 
sheltering  gonotheca,  as  shown  in  the  figure. 
^  The  colony  itself  increases  in  size  by  the  formation  of  new 
buds  in  regular  succession,  alternately  on  the  right  and  left  sides, 
beneath  what  is,  for  the  time  being,  the  topmost  hydranth  of  each 
main  stem,  each  new  bud  giving  rise  to  a  hydranth-bearing 
branch  which  overtops  its  immediate  predecessor.  The  buds 
formed  on  the  blastostyle,  on  the  other  hand,  do  not  develop  into 
KyHranths  at  all,  but  into  another  kind  of  individual  known  as 
a  .medusa,  or  medusoid  person,  which  presently  detaches  itself 
from  the  parent  and  escapes  through  the  mouth  of  the  gonotheca 
as  a  free-swimming  individual. 

The  medusa  of  Obelia  (Fig.  60,  B),  though  larger  than  the 
hydranths,  is  still  very  small,  not  more  than  about  T\jth  of  an 
inch  in  diameter.  At  first  sight  it  looks  very  different  from 
a  hydroid  individual  (hydranth),  but  it  can  easily  be  shown  to 
have  the  same  fundamental  plan  of  structure.  It  consists  of  a 
circular  disk  surrounded  by  a  fringe  of  tentacles  (ten.).  From  the 
middle  of  one  surface  projects  a  handle-shaped  structure,  the 
manubrium  (wro.),  corresponding  to  the  hypostome  of  the  hydranth 
and  bearing  the  mouth  at  its  extremity.  This  mouth  leads  into 
a  central  digestive  cavity  from  which  four  radial  canals  (r.c.)  run 
outwards  through  the  gelatinous  ground-substance  to  a  circular 
marginal  canal.  The  medusa  swims  actively  about  by  muscular 
contractions  of  the  disk,  and  in  doing  so  it  assumes  a  bell-like 
shape  with  the  manubrium  projecting  from  the  convex  surface, 
like  the  handle  of  the  bell. 

Most   medusa — the   larger  of  which   are   familiar   to   us    as 


ALTERNATION   OF   GENERATIONS  121 

"jelly-fish" — are  more  definitely  bell-shaped,  but  the  manu- 
brium  lies  in  the  hollow  of  the  bell,  like  its  clapper ;  in  comparison 
with  these  the  Obelia  medusa  may  be  said  to  turn  itself  inside  out 
in  the  act  of  swimming.  In  accordance  with  their  active  life  the 
medusae  have  a  fairly  well  developed  nervous  system  and  a  number 
of  sense  organs  around  the  margin  of  the  bell.  The  point  with 
which  we  are  immediately  concerned,  however,  is  their  mode  of 
reproduction^  ]JLu*v  J$* 

Unlike  the  hydrajiths  and  blastostyles  of  which  the  Obelia 
colony  is  composed  the  medusae  are  sexual  individuals,  bearing 
sexual  organs  or  gonads  from  which  the  gametes  are  set  free. 
The  gonads  (Fig.  60,  B,  gon.)  are  masses  of  germ  cells  lying  one 
beneath  each  radial  canal,  on  the  same  side  of  the  disk  as  the 
manubrium.  The  sexes  are  distinct,  the  medusae  being  either 
male  or  female,  and  the  gonads  accordingly  producing  either 
spermatozoa  or  ova,  which,  when  mature,  are  discharged  into  the 
water  by  rupture  of  the  gonad. 

The  ova  are  fertilized  by  the  spermatozoa  and  undergo 
segmentation  much  as  in  the  case  of  Hydra,  developing 
ultimately  into  a  hydroid  individual  (hydranth),  which  by 
budding  gives  rise  to  a  new  Obelia  colony. 

It  is  obvious  that  we  have  here  a  case  of  alternation  of  sexual 
and  asexual  generations  (metagenesis),  comparable  to  that  which 
occurs  in  the  fern,  except  that  the  sexual  generation  arises  from  a 
multicellular  bud  and  not  from  a  unicellular  spore.  The  sexual 
medusot3^individuaris  produced  by  budding  from  the  asexual 
hydroid,  and  itself  gives  rise,  through  the  conjugation  of  gametes 
and  the  development  of  the  zygote  thus  produced,  to  the  asexual 
hydroid  again.  The  process  is  somewhat  complicated  by  the  fact 
that  the  hydroid  alsq,  produces  other  asexual  individuals  by  bud- 
ding, but  this  really  amounts  to  little  more  than  does  the  forma- 
tion of  numerous  branches  by  the  budding  of  one  of  the  higher 
plants,  for  in  the  latter  case  also  each  bud  may  be  regarded  as, 
potentially  at  any  rate,  a  perfect  individual^ fio^p**^ 

We  may  draw  an  even  closerpeHiparison  between  the  alterna- 
tion of  hydroid  and  medusoid>fenerations  in  the  Coelenterata  and 
that  of  sporophyte  and  gametophyte  in  the  higher  plants,  for  in  the 
former,  as  in  the  latter,  we  see  in  many  instances  a  more  or  less 
strongly  developed  tendency  towards  the  suppression  of  the  sexual 
generation  (sometimes  called  the  gamobium  in  animals). 

In   Tubularia,   for   example,   the   medusoid   individual   never 


122        OUTLINES   OF  E VOLUTION AEY   BIOLOGY 

separates  from  the  parent  colony,  but  remains  attached  to  it  as  a 
bud,  though  still  showing  clear  evidence,  in  its  bell-like  shape  and 
in  the  presence  of  the  manubrium  and  of  radial  and  circular 
canals,  of  its  medusoid  nature.  It  is  in  fact  merely  a  degenerate 


art.' 


fen 


FIG.  61. — A  single  Hydranth  of  Tubularia,  with  medusoid  Individuals 
(Gonophores)  budded  out  between  the  two  Circles  of  Tentacles;  highly 
magnified.  (After  Allman.) 

act.,  actinula  larva  enclosed  in  gonophore;  act.',  actinula  larva  escaping;  ent.,  enteron 
or  digestive  cavity  ;  gon.,  gonopliore;  m.,  mouth;  per.,  perisarc;  ten.,  tentacles. 

medusa  or  gonophore  (Fig.  61,  gon.).  It  still  retains  its  sexual 
function,  producing  either  ova  or  spermatozoa,  and  in  this  case 
the  fertilized  ova  actually  develop  into  young  hydroids  or 
"  actinula  larvae  "  (Fig.  61,  act.,  act.')  before  escaping  from  the 
parent  gonophore.  We  have  here  a  telescoping  of  the  successive 
generations  quite  comparable  to  what  we  find  in  a  flowering 


SUPPRESSION   OF  MEDUSOID  123 

plant,  and  again  it  is  the  sexual  generation  which  has  undergone 
reduction. 

In  other  cases  the  medusoid  bud  or  gonophore  more  or  less 
completely  loses  its  medusa-like  structure  and  becomes  a  mere 
sac  containing  the  gonad,  and  it  has  been  suggested  that  in 
Hydra  we  may  have  the  last  stage  in  this  progressive  reduction 
of  the  sexual  generation,  the  medusoid  having  altogether  dis- 
appeared, after  having  transferred  its  sexual  functions  to  the 
hydroid. 

It  seems  almost  incredible  that  the  germ  cells  themselves 
which  are  originally  produced  by  one  generation  should  in  this 
manner  be  transferred  to  what  is  really  the  preceding  generation, 
but  as  a  matter  of  fact  we  see  all  stages  in  this  process  in  different 
genera  of  Hydrozoa.  Even  in  Obelia  and  certain  other  medusae 
the  germ  cells  do  not  originate  in  the  gonads  beneath  the  radial 
canals  but  in  the  ectoderm  of  the  manubrium,  reaching  their 
final  position  by  a  process  of  migration.  In  Eudendrium,  where 
the  medusoid  is  reduced  to  a  mere  vestige,  the  germ  cells 
no  longer  originate  in  the  sexual  individual  at  all,  but  in  the  main 
stem  of  the  hydroid  colony,  whence  they  migrate  into  the 
medusoid  bud.  If  we  imagine  them  ceasing  to  migrate  from  their 
place  of  origin  in  the  hydroid,  while  the  medusoid  is  no  longer 
formed,  we  fe;ach  the  condition  of  Hydra,  with  a  complete  trans- 
ference of  the^  sexual  function  to  what  should  be  the  asexual 
generation. 

In  the  coelenterates,  however,  it  is   not  always  the  asexual 

(hydroid)  generation  which  predominates,  for  in  some  cases  this 

^suppressed  more  or  less  completely  and  the  medusa  or  jelly  - 

fish  reproduces  its  own  kind  directly  without  the  intervention  of 

a  hydroid  phase. 

In  coelenterate  animals  such  as  Hydra  and  Obelia,  as  we 
have  already  seen,  the  body  consists  of  a  single  hollow  tube 
whose  wall  is  made  up  of  only  two  cell  layers,  the  ectoderm 
on  the  surface  and  the  endoderm  lining  the  digestive  cavity,  with 
a  supporting  layer  of  gelatinous  consistency — the  mesogloea 
— between  the  two.  In  the  case  of  the  larger  jelly-fish 
this  mesogloea  attains  a  great  thickness ;  it  is,  however, 
never  a  true  cell  layer  like  the  ectoderm  and  endoderm,  but 
rather  of  the  nature  of  intercellular  substance  secreted  by  the 
cells  on  either  side  of  it. 

In  Hydra  and  Obelia  the  germ  cells  arise  from  the  ectoderm, 


124       OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

and  the  gonads  (Fig.  62,  A,  gon.)  are  accordingly  situated  close 
to  the  surface  of  the  body  and  no  special  ducts  are  required  for 
the  conveyance  of  ova  or  spermatozoa  to  the  exterior. 
•  In  the  great  group  Coelomata,  which  includes  practically  all 
animals  higher  in  the  scale  of  organization  than  the  coelenterates, 
we  find  a  different  state  of  affairs.  The  mesogloea  has  been 
replaced  by  a  true  cellular  mesoderm  (Fig.  62,  B,  mes.),  formed  of 
cells  derived  from  ectoderm  or  endoderm,  or  from  both,  and  in 
the  thickness  of  this  layer  a  cavity  is  developed,  the  coslom  or 


CfOTV. 


A. 


FIG.  62. — Comparison  of  Coelenterate  and  Invertebrate  Coelomate  Types  of 

Structure. 

A.  Diagram  of  a  transverse  section  of  a  coelenterate  animal. 

B.  Diagram  of  a  transverse  section  of  an  invertebrate  ccelomate  animal. 

ccel.  ccelom  or  body  cavity;  cod,  ep.  coelomic  epithelium  lining  body  cavity;  d.v.  dorsal 
blood-vessel ;  ect.  ectoderm ;  end.  endoderm ;  ent.  enteron  or  digestive  cavity ; 
g.a.  genital  aperture ;  g.d.  genital  duct  (gonoduct) ;  gon.  gonad ;  mes.  mesoderm ; 
mesgl.  mesogloea ;  n.c.  double  nerve  cord ;  sow.  somatopleure  or  body  wall ;  spl. 
splanchnopleure  or  gut  wall ;  v.v.  ventral  blood-vessel. 

body  cavity  (coel.),  which  more  or  less  completely  surrounds  the 
digestive  tube  or  alimentary  canal  (compare  the  development  of 
Amphioxus,  Fig.  13,  XI — XIII) .  The  outer  layer  of  the  mesoderm 
unites  with  the  ectoderm  to  form  the  body  wall  or  somatopleurS 
(som.) ,  while  the  inner  layer  unites  with  the  endoderm  to  form  the 
gut  wall  or  splanchnopleure  (spl.),  and  the  body  thus  acquires  the 
form  of  a  double  tube.  The  body  cavity  is  lined  by  a  layer  of 
epithelial  cells  (coel.  ep.),  and  it  is  from  this  layer  that  the  germ 
cells  apparently  arise.  The  gonads  (Fig.  62,  B,  gon.)  therefore 
project  into  the  body  cavity  and  into  this  cavity  the  mature  germ 
cells  are  primarily  discharged.  In  the  great  majority  of  cases 


SECONDARY   SEXUAL   CHARACTERS  125 

special  genital  ducts  or  gonoducts  (g.d.)  are  developed  which 
pierce  the  body  wall  and  serve  for  the  passage  of  the  germ  cells 
to  the  exterior.  The  sexes  are  usually  distinct  in  the  higher 
forms  (Vertebrata),  but  many  of  the  invertebrate  coelomates 
(e.g.,  the  earthworm)  are  hermaphrodite,  the  same  individual 
bearing  both  male  and  female  gonads  (testes  and  ovaries)  with 
the  corresponding  gonoducts  (vasa  deferentia  and  oviducts). 

Fertilization  of  the  ova  by  the  spermatozoa,  or  in  other  words 
conjugation  of  the  gametes,  may  take  place  either  within  the 
body  of  the  parent,  as  in  most  terrestrial  forms,  or  externally,  as 
in  a  very  large  proportion  of  aquatic  animals.  In  the  former  case 
special  organs  are  developed  for  the  transference  of  the  sperma- 
tozoa from  one  individual  to  another,  and  such  transference 
usually  occurs  even  in  hermaphrodite  forms,  which,  as  a  rule,  are 
incapable  of  self-fertilization.  Further  modifications  may  arise 
in  connection  with  the  nutrition  of  the  embryo,  which  may 
remain  within  the  body  of  the  parent — in  an  enlarged  portion  of 
the  oviduct  known  as  the  uterus — until  it  has  reached  an 
advanced  stage  of  development.  This  takes  place  more  par- 
ticularly in  the  females  of  the  higher  vertebrates. 

In     connection     with    the     sexual     differentiation,      more 
especially  in  the  higher   animals',  numerous   secondary  sexual  \ 
characters  may  arise  which  are  not  directly  connected  with  the  ^ 
organs  of  reproduction.     Such  are  the  various  ornamental  out- 
growths of  hair,  feathers  and  so  forth,  which  distinguish  the  males 
of  many  vertebrates  and  are  supposed  to  appeal  to  the  aesthetic 
sense   and   thus  to  contribute   towards  the  mutual  attraction 
between  male   and  female,  and   the   special  weapons,   such  as 
antlers  and  spurs,  which  male  animals  frequently  develop  and. 
which  are  used  in  combat  for  the  possession  of  the  females. 

Although  not  directly  connected  with  the  gonads  these 
secondary  sexual  characters  seem  to  depend  for  their  development 
in  some  curious  way  upon  the  presence  of  these  organs.  Thus 
it  is  well  known  that  if  the  testes  be  removed  by  castration  the 
secondary  sexual  characters  will  not,  in  most  cases  at  any  rate, 
develop  properly.  We  see  an  excellent  illustration  of  this  in  the 
case  of  the  antlers  of  the  stag,  which  are  confined  to  the  male  and 
do  not  develop  at  all  if  the  animal  be  castrated  in  early  youth, 
while  if  the  operation  be  performed  after  the  antlers  are  fully 
developed  these  are  prematurely  cast  off  and  replaced  by 
imperfect  ones. 


Ill  I 

/nil 


126        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

How  this  intimate  correlation  between  gonads  and  secondary 
sexual  characters  is  brought  about  is  still  uncertain,  but  there  is 
strong  reason  to  suppose  that  it  is  due  to  the  secretion  by  the 
gonad  of  some  specific  substance  (hormone),  perhaps  of  the 
nature  of  a  ferment,  which  circulates  throughout  the  body- 
chiefly  no  doubt  in  the  blood — and  controls  the  development 
of  the  characters  in  question.  That  internal  secretions  may  act 
in  this  way  upon  organs  remote  from  their  own  place  of  origin  is 
well  known  and  is  strikingly  exemplified  in  the  case  of  the 
corpora  lutea  of  the  mammalian  ovary.  These  structures  appear 
on  the  surface  of  the  ovary  in  the  places  whence  ova  have  been 
discharged,  and  are  apparently  of  a  glandular  nature.  As  the 
fertilized  ova  pass  down  the  oviduct  they  begin  to  develop,  and 
-on  reaching  the  uterus  fix  themselves  to  the  wall  of  the  latter,  in 
which  they  become  imbedded.  The  spot  where  fixation  takes  place 
is  far  distant  from  the  ovary,  but  it  has  been  demonstrated  that 
if  the  corpora  lutea  on  the  surface  of  the  latter  be  destroyed  the 
embryos  will  not  become  fixed  at  all.  There  is  a  close  correlation, 
then,  between  the  presence  of  the  corpus  luteum  and  the  fixation  of 
the  embryo,  and.  this  is  explained  by  supposing  that  the  corpus 
luteum  secretes  some  substance  which,  circulating  in  the  blood, 
reaches  the  uterus  and  stimulates  its  epithelial  lining  to  respond 
to  contact  with  the  embryo.  Moreover,  the  reaction,  whatever  its 
cause,  appears  to  be  mutual,  for  if  the  discharged  ovum  does  not 
get  fertilized  the  corpus  luteum  does  not  attain  its  full  develop- 
ment and  soon  disappears. 

We  have  now  very  briefly  traced  the  evolution  of  sexual 
characters  from  their  starting  point  in  the  male  and  female 
gametes  of  the  Protista  to  their  culmination  in  the  higher  plants 
and  animals.  We  have  seen  how  sexual  differentiation,  which 
primarily  concerns  the  gametes  themselves,  is  gradually  extended 
to  the  colonies  or  to  the  multicellular  individuals  from  which  the 
gametes  arise,  or  even  to  a  preceding,  originally  non-sexual 
generation.  The  various  structural  modifications  thus  brought 
about  are  all  directed  towards  one  end,  the  conjugation  of  the 
gametes.  The  mutual  attraction  which  undoubtedly  exists 
between  the  gametes  themselves  is  not  sufficient,  at  any  rate  in 
the  case  of  the  more  highly  developed  and  complex  organisms, 
where  the  distances  between  their  places  of  origin  are  relatively 
very  great,  to  secure  their  union. 

In  the  flowering  plants  their  own  efforts  are  supplemented  by 


THE   EVOLUTION   OF   SEX  127 

all  the  elaborate  devices  for  securing  pollination,  and  by  far  the 
most  active  part  in  the  process  is  played  by  external  agencies, 
especially  by  those  insects  which  have  become  the  vicarious 
fertilizers  of  the  flowers. 

In  the  higher  animals,  on  the  other  hand,  the  necessity  for 
bringing  the~ganiete's  into  close  proximity  with  one  another  has 
led  to  the  development  of  all  those  secondary  sexual  characters,  both 
bodily  and  mental,  which  play  so  conspicuous  a  part  in  the  drama 
of  life.  Throughout  the  whole  course  of  this  remarkable  process 
of  evolution,  except  in  the  case  of  certain  obviously  degenerate 
forms,  we  observe  that  same  fundamental  distinction  between  the 
sexes  which  we  first  noticed  in  the  gametes  of  unicellular 
organisms,  and  which  in  the  higher  animals  is  extended  with 
the  sexual  differentiation  itself  from  the  gametes  to  the  complex 
multicellular  body  which  bears  them.  The  female  is  the  more 
passive  partner  and  is  especially  concerned  with  the  nutrition 
and  rearing  of  the  offspring,  and  her  bodily  organization  is 
especially  adapted  to  her  maternal  functions.  These  functions 
constitute  an  inevitable  handicap  in  the  struggle  for  existence, 
and  the  females  and  young  of  the  higher  animals  are  in  most 
cases  largely  dependent  upon  the  less  burdened  and  consequently 
more  active  and  vigorous  males  for  their  protection. 

The  explanation  of  this  progressive  sexual  differentiation  is 
undoubtedly  to  be  found  in  the  advantages  to  be  derived  from 
division  of  labour  and  the  accompanying  possibilities  of  special- 
ization. The  origin  of  conjugation  itself,  upon  which  all  sexual 
phenomena  are  based,  is  another,  and  more  fundamental,  questioriSw 
At  first,  as  we  have  seen,  the  conjugating  gametes  were  apparently  ^ 
exactly  alike  one  another  and  exhibited  no  visible  sexual  differen- 
tiation at  all.  The  habit  of  conjugation  probably  arose  from 
the  necessity  of  making  good  some  disturbance  of  equilibrium  in 
the  protoplasm  of  the  cell.  It  has  been  supposed  that,  as  the  result 
of  repeated  fission,  some  condition  of  inequality  was  gradually 
set  up  amongst  the  daughter  cells,  whereby  some  of  them  came 
to  have  too  much  of  one  constituent  and  too  little  of  another, 
while  others  were  in  the  opposite  condition.  In  this  way  the 
successive  unicellular  generations  gradually  became  more  and 
more  enfeebled — as  we  saw  in  the  case  of  Paramoecium — and,  owing 
perhaps  to  some  sort  of  polarization,  those  which  had  become 
modified  in  opposite  directions  came  to  exercise  an  attraction 
upon  one  another  which  resulted  in  conjugation  and  restoration 


128        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

of  the  proper  equilibrium.  It  may  be  that  from  the  very  first 
the  inequalities  of  fission  resulted  in  the  accumulation  of  more 
active  protoplasm  in  some  cells  and  a  greater  amount  of  reserve 
material  in  others,  and  that  this  was  the  starting  point  of  the 
differentiation  into  male  and  female. 

Although  fusion  of  the  nuclei  (karyogamy)  of  the  two 
gametes  appears  now  to  be  the  most  important  feature  of  con- 
jugation, we  must  suppose  that  it  was  preceded  by  plastogamy1  or 
fusion  of  the  cytoplasm,  which  obviously  constitutes  the  natural 
preliminary  to  karyogamy.  In  some  of  the  lower  Protozoa  such 
plastogamy  has  been  sometimes  observed  unaccompanied  by 
karyogamy,  and  it  is  possible  that  in  some  cases  plastogamy 
alone  is  sufficient  to  bring  about  rejuvenescence  and  renewed 
activity  in  cell-division. 

1  Sometimes  called  plasmogamy. 


CHAPTER  X 

Origin  of  the  germ  cells  in  nralticellular  animals  —  Maturation  of  the  germ 
cells  —  Eeduction  of  the  chromosomes  —  Sex  determination  in  insects  — 
Different  forms  of  gametes  —  Mutual  attraction  of  the  gametes  — 
Fertilization  and  parthenogenesis. 


IN  many  multicellular  animals  the  distinction  between  somatic 
cells  and  germ  cells  becomes  manifest  at  a  very  early  stage  in  the 
development  of  the  individual.  An  extreme  instance  of  this  is 
seen  in  the  parasitic  round-worm  of  the  horse,  Ascaris  megalo- 
cepliala.  Here  the  distinction  in  question  precedes  all  other 
histological  differentiation.  The  two  cells  or  blastomeres  into 
which  the  fertilized  ovum  first  divides  (Fig.  35,  C,  D)  are  originally 
similar  to  one  another,  but  as  they  prepare  for  the  next  mitotie 
division  of  the  nucleus  a  remarkable  difference  is,  according  to 
the  observations  of  Professor  Boveri,  established  between  them. 
Both  at  first  (in  the  case  of  the  variety  known  as  univalens) 
exhibit  two  elongated  chromosomes  (the  variety  bivalens,  which  ia 
representeST  in  Fig.  35,having  four),  but  in  one  the  thickened 
ends  of  the  two  chromosomes  are  thrown  off  into  the  surrounding1 
cytoplasnTj^vhere  IheyTfegenerate,  while  the  mofe~slender  middle 
portions  break  up.  into  a  number  of  short  pieces.  Thus  two 
differentiated  cells  are  produced,  one  with  two  large  chromosomes 
and  the  other  with  numerous  small  ones.  The  latter  gives  rises 
by  its  subsequent  divisions  to  somatic  cells  only.  The  former  is 
a  primordial  germ  cell  ;  for  some  five  or  six  times  it  will  divide 
like  its  parent  cell  into  a  somatic  cell  and  a  primordial  germ  cell, 
but  after  these  early  divisions  the  primordial  germ  cells  will 
give  rise  to  their  own  kind  only,  until  the  time  comes  for  the 
production  of  the  actual  gametes.  The  somatic  cells,  on  the 
other  hand,  will  gradually  become  differentiated  into  all  the 
various  tissue  cells  of  the  adult. 

Perhaps  the  most  significant  part  of  this  remarkable  process 
as  observed  in  Ascaris  is  the  elimination  of  chromatin  material 
from  the  nuclei  of  the  somatic  cells  when  these  are  firgi 

B.  K 


130        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


differentiated  from  the  germ  cells.  The  result  of  this  is  that  the 
germ  cells  alone  retain  the  full  complement  of  chrornatin 
derived  from  the  parents,  and  their  nuclei  are  accordingly 
actually  much  larger  than  those  of  the  somatic  cells. 

The  differentiation  into  somatic  cells  and  germ  cells  cannot 
usually  be  traced  so  far  back  in  the  development  of  the  individual 
as  in  Ascaris,  but  in  a  great  many  animals  the  distinction  can 
be  recognized  at  a  very  early  stage.  In  certain  insects,  for 
example,  the  primordial  germ  cells  can  be  traced  back  to  a  large 
"  pole-cell "  which  lies  at  one  end  of  the  segmenting  ovum,  and 

in  the  arrow-worm,  Sagitta, 
they  can  be  identified  at  the 
gastrula  stage  (Fig.  63,  p.g.c.). 
As  we  shall  see  later  on,  this 
early  segregation  of  the  germ 
cells  is  of  very  great  interest 
from  the  point  of  view  of  the 
theory  of  heredity.  It  can 
hardly  be  said,  however,  at 
any  rate  in  the  present  state 
of  our  knowledge,  to  be  a 
phenomenon  of  universal  or 
even  general  occurrence,  and 
in  the  majority  of  coelomate 
animals  the  germ  cells  are  first 
recognizable  in  the.  coelomic 
epithelium  at  a  compara- 
tively late  stage  of  develop- 
In  plants  also,  in  cases  where  there  is 

a  well  developed  gametophyte  this  appears  to  attain  its  full 
development  before  the  germ  cells  are  recognizable,  and  the 
entire  life  of  the  sporophyte  is  passed  without  any  distinction 
between  somatic  and  germ  cells  manifesting  itself.  Moreover, 
the  fact  that  the  ordinary  cells  of  a  fern  prothaljua.  can,  on 
occasion,  act  as  germ  cells,1  prevents  us  from  admitting  any 
absolute  distinction  between  the  two  categories. 

The  primordial  germ  cells  may  undergo  extensive  multiplication 
by  ordinary  mitotic  division  before  giving  rise  to  the  actual 
gametes.  In  multicellular  animals  the  process  of  gametogenesis 
(formation  of  gametes)  which  is  either  oogenesis  or  spermato- 

i  Vide,  p.  105. 


FIG.  63.— Section  of  the  Gastrula  of  an 

Arrow  Worm  (Sagitta)  showing  the 
™:_^;~i  n_-_-  r.^      (Mi&T  Q. 

;  ep.,  epiblast; 
;imordial  germ 


MALE   AND   FEMALE    PRONUCLEI 


183 


genesis  according  to  whether  ova  or  spermatozoa  are  produced 
thereby,  is  accompanied  by  nuclear  phenomena  of  very  great 
interest,  whereby  the  maturation  of  the  germ  cells  is  effected^ 
Before  describkjg  this  process  we  must  lay  stress 
preliminary  conaiderations 

KfT  w<3  have  already  seen,  each  kind  of  animal  or  plant  is 
characterized  by  the  appearance  of  a  definite  number  of  chromo 
somes  in  the  nuclei  of  its  cells  at  the  time  when  these  are  under- 
going division  by  mitosis.  Althou^l^  not  absolutely  constant  in 
all  cases  the  number  is  usually  the  same  for  all  the  different 
somatic  cells  of  which  the  body 
is  composed.  It  is  usually  an 
even  number,  and  (with  certain 
exceptions)  it  remains  the  same 
in  successive  generations  of 
individuals. 

It  will  also  be  remembered 
that  the  zygote  or  fertilized 
egg  from  which  the  individual 
develops  is  formed  by  the  con- 
jugation of  two  gametes,  ovum 
and  spermatozoon,  and  that  in 
this  process  the  nuclei  of  the 
gametes,  sometimes  called  the 
male  and  female  pronuclei, 
unite,  or  at  any  rate  co-operate 
as  a  single  nucleus.  Fig.  64  is 
taken  from  an  actual  photograph  of  an  egg  of  Ascaris  in  process 
of  fertilization  ;  the  spermatozoon  has  already  entered  the 
ovum  and  the  male  and  female  pronuclei  (pro.)  are  seen  lying 
side  by  side  in  the  cytoplasm.  Each  pronucleus  brings  with  it 
its  own  set  of  chromosomes,  and  hence  the  zygote  nucleus  has 
double  the  number  of  chromosomes  possessed  by  either  of  the 
gametes.  Thus  it  appears  at  first  .sight  that  every  conjugation 
or  sexual  union  of  gametes  (zygosis)  must  be  accompanied 
by  a  doubling  of  the  number  of  chromosomes,  and  we  might 
therefore  expect  to  find  each  successive  generation  with  twice  as 
many  chromosomes  in  its  nuclei  as  the  preceding  one.  That 
this  is  not  actually  so  depends  (in  the  case  of  animals)  upon  the 
fact  that  the  nuclei  of  the  gametes  contain  only  half  the  number 
of  chromosomes  characteristic  of  the  somatic  cells ;  if  the 

K  2 


Fio.  64*— Ovum  of  the  Horse  Worm 
(Ascaris  megalocephala]  during 
the  Process  of  Fertilization, 
showing  the  male  and  female 
Pronuclei  (pro),  X  770.  (From 
a  photograph.) 


130 


NES   OF   EVOLUTIONARY  BIOLOGY 


have  eight  the  mature  ovum  or  spermatozoon  will 
A  four,  and  so  on.     This  reduction  of  the  number  of 
/omes  (meiosis)  is  the  essential  part  of  the  process  of 
/ation  which  the  animal   germ   cells    undergo,  and  it   is 


Spermalogenes 
I 


Spermatogonia    Q   , 


Oogenesis 


Multiplication  of  Epithelial 
Cells  ( Spermatogonia  and 
Oogonia)  in  the  Gonad 
(Test is  or  Ovary)  by  ordinary 
Somatic   Mitosis' 


?  Oogonfo 


Male     Gametes 


Synapsis  or  Pairing    A 
of  the  Chromosomes  \, 

h                ( 

-^Primary 
S)0ocytes 

\ 
Reducing   Division        ! 
(Meiosis  )            ; 

§                 F 

K             f 

>l>  Secondary  Oocytes 
V  ^flo'  /^fe/1  fioc//f5 

\                                i|  , 

; 

Polar    IL& 
s    X                 **te»Jr 
)   (£                         && 

-&>'i   M?^^f  «7w 
,7°^  Polar  Bodies 

ii     ' 

I       i    ^Conjugation  of  .' 

'           ^fertilization/ 
\ofOvitm/' 

b   c 

4  j  Mature  Ovc 
lameces 

)  Zygote  or  Fertilized  Ovum 


C*L\jp  (The  Figures  indicate  the  numbers 
•  Segmentation  of      ;'(    ;;      of  Chromosomes  present) 
the  Zygote   by          M  }• 
Ordinary  Somatic  @@ 
Mitosis  ?»¥«) 


FIG.  65. — Diagram  of  Gametogenesis. 

effected  by  a  modification  of  the  mitotic  nuclear  division,  in 
which  the  chromosomes  are  separated  into  two  groups,  half  the 
total  number  of  entire  chromosomes  going  into  one  daughter 
cell  and  half  into  the  other. 

In  a   typical  case  of   spermatogenesis  the  first  stage  is  the 


GAMETOGENESIS  133 

multiplication  of  cells  (the  so-called  spermatogonia)  derived  from 
the  germinal  epithelium,  in  the  testis,  by  ordinary  cell-division. 
Let  us  suppose  the  number  of  chromosomes  found  in  the  nuclei 
of  the  somatic  cells  to  be  eight ;  it  will,  of  course,  remain  the 
same  so  long  as  the  character  of  the  mitosis  undergoes  no 
change,  each  chromosome  splitting  into  two  at  every  nuclear 
division.  Presently,  however,  we  find  the  chromosomes  arranged 
in  pairs  instead  of  all  appearing  separately  in  the  mitotic  figure, 
and  the  cells,  which  have  increased  considerably  in  size  by  the 
absorption  of  nutriment,  may  now  be  termed  primary  spermato- 
cytes.  This  pairing  of  the  chromosomes  (synapsia)_jiiarks  the 
onset  of  the  "reducing  division  " ;  a  nuclear  spindle  is  formed,  the 
.paired  chromosomes  arrange  themselves  upon  it,  and  the  two 
members  of  each  pair  separate  and  travel  towards  opposite  poles. 
'Thus  two  new  nuclei  are  formed  each  with  only  four  chromosomes. 
iThe  reduction  is  now  complete  and  the  new  generation  of  cells, 
with  reduced  nuclei,  may  be  termed  secondary  spermatocytes. 
One  more  mitotic  division  takes  place,  this  time  involving  the 
^splitting  of  each  chromosome,  so  that  there  is  no  further  reduc- 
tion in  their  number,  and  giving  rise  to  the  minute  spermatids, 
each  of  which  develops  a  long,  vibratile,  cytoplasmic  tail  and 
forms  a  spermatozoon.  Hence  we  see  that  each  primary 
spermatocyte  gives  rise  to  four  spermatozoa,  with  reduced 
nuclei  containing  half  the  number  of  chromosomes  found  in  the 
somatic  cells.  The  essential  features  of  the  whole  process  are 
represented  diagrammatically  in  Fig.  65. 

The  process  of  oogenesis  takes  place  in  essentially  the  same 
manner ;  the  so-called  oogonia,1  derived  from  the  germinal 
epithelium  of  the  ovary,  multiply  and  give  rise  to  oocytes. 
Synapsis  and  reduction  in  the  number  of  the  chromosomes  take 
place  as  they  do  in  spermatogenesis,  but,  owing  doubtless  to  the 
fact  that  it  takes  a  comparatively  large  amount  of  cytoplasm  to 
form  the  body  of  an  egg,  we  find  that  only  one  perfect  ovum 
arises  from  each  primary  oocyte,  the  other  three  forming  the 
"  polar  bodies."  Owing  to  their  minute  size  as  compared  with 
the  ovum  itself,  two  of  the  polar  bodies  (Fig.  35,  A,  p.b.)  appear 
to  be  cast  out  of  the  latter  as  it  undergoes  maturation,  while 
the  third  is  formed  by  division  of  the  first ;  it  will  be  clear, 
however,  from  a  careful  study  of  Figs.  65  and  68  that  the 

1  This  term  is  very  unfortunately  chosen  and  must  not,  of  course,  be  confounded 
with  the  same  term  as  applied  to  the  female  organs  of  such  plants  as  Fucus. 


134        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


FIG.  66. — Some  Stages  in  the  Spermatogeuesici  01  a  urasshopper  (Stenolothrus 
viridulus).     (After  Meek.) 

1.  A  secondary  spermatogonium  in  mitosis,  with  seventeen  chromosomes  (eight  pairs  and 

one  accessory).     Polar  view. 

2.  Resting  or  growth  stage  following  the  mitosis  of  the  secondary  spermatogonium.     The 

ordinary  chromosomes   have   become   resolved   into  a  network  but   the  accessory 
chromosome  retains  its  individuality. 

3.  Primary  spermatocyte  formed  by  growth  of  2,  preparing  for  mitosis. 

4.  Mitosis  of  the  primary  spermatocyte,  showing  the  pairing  (synapsis)  of  the  sixteen 

ordinary  chromosomes   on    the    nuclear    spindle   and   the   accessory   chromosome 
unpaired.     Side  view. 

6.  Later  stage  in  the  mitosis  of  the  primary  spermatocyte,  showing  separation  of  the 
paired  chromosomes.     Side  view. 

6.  Still  later  stage  in  the  division  of  the  primary  spermatocyte  into  two  secondary  sper- 

matocytes ;  the  chromosomes  massing  at  the  two  poles  of  the  spindle  (one  mass  only 
will  contain  the  accessory  chromosome).     Side  view. 

7.  A  secondary  spermatocyte  in  mitosis,  showing  the  reduced  number  of  ordinary  chromo- 

somes (S)  and  the  accessory  chromosome.    (The  sister-cell  would  of  course  contain  no  ' 
accessory  chromosome).     Polar  view. 

8.  Later  stage  in  the  mitosis  of  the  secondary  spermatocyte  ;  each  chromosome  (including 

the  accessory  one,  which  lags  behind  the  others)  has  split  into  two  parts  and  the  two 
groups  are  separating.     Side  view. 

9.  A   spermatid,  formed  by  division   of   a   secondary  spermatocyte  with  an  accessory 

chromosome.  •  It  will  form  a  single  spermatozoon. 

x   The  accessory  chromosome. 


I 


SPEEMATOGENESIS   IN   INSECTS  135 

process  is  merely  one  of  repeated  cell-division  as  in  the  case  of 
spermatogenesis.  The  three  polar  bodies  consist  almost  entirely 
of  chromatin  and  each  of  course  contains  the  same  reduced 
number  of  chromosomes  as  the  ovum  itself;  they  undergo  no 
further  development,  however,  and  finally  disappear.  The  truth 
of  the  view,  now  generally  held,  that  the  polar  bodies  are 
merely  ova  which  have  not  sufficient  cytoplasm  to  allow  of  their 
development,  is  demonstrated  by  the  fact  that  in  one  of  the 
turbellarian  flat-worms,  according  to  Francotte,  the  exceptionally 
large  first  polar  body  may  occasionally  be  fertilized  and  actually 
develop  as  far  as  the  gastrula  stage. 

The  details  of  the  process  of  gametogenesis  vary  very  much 
in  different  cases,  but  the  above  outline  may  be  regarded  as 
generally  applicable. 

In  a  large  number  of  insects  it  has  been  found  that  the  male 
animal  possesses  an  odd  number  of  chromosomes  in  the  somatic 
nuclei,  due  to  the  presence  of  a  single  "accessory"  chromosome 
or  "  monosome,"  while  the  female  possesses  an  even  number 
(one  more  than  the  male  owing  to  the  presence  of  two 
"  accessory  "  chromosomes).  This  leads  to  a  curious  complica- 
tion in  the  process  of  spermatogenesis.  The  accessory  chromo- 
some (Fig.  66,  X)  can  often  be  distinguished  by  its  appearance 
from  the  others,  and  at  the  time  of  synapsis  (Fig,  66,  4)  it  has 
/no  mate.  Hence  in  the  reducing  division  the  chromosomes  are 
[  separated  into  two  unequal' groups,  one  of  which  contains  the 
accessory  chromosome  while  the  other  does  not  (Fig.  66, 5).  Two 
kinds  of  spermatozoa  are  accordingly  produced  in  equal  numbers, 
one  kind  with  an  odd  number  of  chromosomes  and  the  other  with 
an  even  number.  * 

The  matured  ova,  on  the  other  hand,  all  have  the  same  number 
of  chromosomes,  because  the  accessory  chromosome  has  a  synaptic 
mate.  Fertilization  of  an  ovum  by  a  spermatozoon  containing 
an  accessory  chromosome  results  in  the  production  of  a  female 
animal  with  an  even  number  of  chromosomes  in  its  somatic  cells  ; 
fertilization  by  a  spermatozoon  which  has  no  accessory  chromo- 
some results  in  the  production  of  a  male  animal  with  an  odd 
number  of  chromosomes  in  its  somatic  cells,  as  shown  in  Fig.  67. 

Thus  it  appears  that  the  chromosomes,  at  any  rate  in  some 
cases,  have  a  very  important  influence  on  the  determination  of 
sex,  and  that  the  latter  is  not,  as  has  often  been  supposed,  merely 
the  result  of  nutritional  and  other  environmental  influences  upon 


136        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

the  developing  organism,  but  a  character  of  far  more  deepl 
seated  origin. 

It  must  not  be  forgotten  that  the  chromosomes,  as  such*  are  / 
only  recognizable  during  the  process  of  mitosis ;  in  the  realm/ 
condition  of  the  nucleus  they  appear  to  be  broken  up  into  latter 
or  smaller  granules  of  chromatin  scattered  through  the  Imin 
reticulum.  The  observations  on  accessory  chromosomes  ablvre 
mentioned,  and  others  to  be  referred  to  presently,  point  to  tne 

d  (Unreduced  Cells )  $  (Unreduced  Cells ) 

(S±l) 


"   (4  +  1) 
9  (  6~~Zygote) 

FIG.  67. — Diagram  illustrating  the  Correlation  between  the  Number  of 
Chromosomes  and  the  Sex  in  certain  Insects.  (The  numbers  of  chromo- 
somes given  in  this  diagram  are  arbitrarily  chosen  and  are  obviously 
different  from  those  which  occur  in  Stenobothrus,  as  shown  in  Fig.  66.) 

conclusion,  however,  that,  in  spite  of  this,  the  different  chromo- 
somes preserve  some  sort  of  individuality  from  one  cell-genera- 
tion to  another.  In  other  words,  we  have  reason  to  believe  that 
th£  chromosomes  which  make  their  appearance  at  the  onset  of 
each  mitosis  are,  taken  each  as  a  whole,  the  same  as  those  which 
become  disintegrated  at  the  close  of  the  preceding  mitosis,  though 
it  is  very  possible  that  the  constituent  parts  of  the  old  chromo- 
somes (chromatin  granules,  chromomeres  or  ids),  after  absorbing 
nutriment  and  increasing  in  size  during  the  resting  period,  may 
come  together  in  new  combinations  to  form  the  new  chromosomes 
each  time  division  of  the  nucleus  takes  place.1 

1  Vide  Farmer,  Croonian  Lecture,  Proe.  Royal  Soc.,  Ser.  B,  Vol.  79  1907. 


PAIRING   OF   CHROMOSOMES  137 

It  is  also  highly  probable,  though  by  no  means  certain,  that  the 
chromosomes  which  are  derived  from  the  male  parent  remain 
throughout  life  distinct  from  those  which  are  contributed  by  the 
female  parent.  According  to  this  view  every  ordinary  somatic 
cell  has  two  sets  of  chromosomes,  paternal  and  maternal  respec- 
tively, and  this  again  is  strongly  supported  by  the  observations 
on  the  germ  cells  of  insects  above  referred  to,  where  all  the 
chromosomes  appear  to  be  duplicated,  with  the  exception  of  the 
accessory  chromosome  in  the  male  animal. 

There  is  reason  for  believing,  therefore,  that  in  ordinary  casek 
every  paternal  chromosome  in  an  unreduced  nucleus  has  an  equiva- 
lent or  "  homologous  "  mate  derived  from  the  female  parent,  and 
that  the  phenomenon  of  synapsis l  represents  a  pairing  of  these 
homologous  paternal  and  maternal  mates.  The__rjdu£irigjlijd£iQn 
which  follows  on  synapsis  consists  in  the  separation  of  the  mates 
once  more,  one  of  each  pair  going  to  each  daughter  cell,  so  that 
the  matured  germ  cells  are  left  with  a  single  instead  of  a  double 
set  of  chromosomes. 

If  we  assume  that,  as  seems  highly  probable,  the  chromo- 
somes of  each  paternal  or  maternal  set  are  not  all  identical 
but  differentiated  amongst  themselves — a  differentiation  which 
in  some  cases  is  actually  visible,  as  shown  in  Fig.  66 — and 
that  one  of  each  kind  is  necessary  to  make  up  the  full  com- 
plement of  the  nucleus  of  the  gamete,  the  importance  of 
the  pairing  of  homologous  chromosomes  which  takes  place  in 
synapsis  becomes  ^a,i  once  evident,  for  one  of  each  pair  goes  to 
each  daughter  nucleus,  which  will  therefore  be  certain  to  receive 
a  chromosome  of  each  kind  instead  of  a  chance  assemblage. 
The  chromosome  of  each  kind  which  it  receives, .-however,  may  be 
either  the  paternal  or  the  maternal  representative  of  that  kind, 
and  as  these,  though  essentially  homologous,  may  differ  from  one 
another  to  some  extent  in  accordance  with  individual  peculiarities 
of  the  parents  from  which  they  were  derived,  it  will  be  seen  that 
the  matured  gametes  may  differ  widely  amongst  themselves  in 
their  nuclear  constitution.  This,  as  we  shall  see  presently,  is  a 
very  important  matter  from  the  point  of  view  of  the  theories 
of  heredity  and  variation. 

This  somewhat  complex  subject  will  be  rendered  more  readily 

1  The  pairing  of  the  chromosomes,  for  which  we  have  used  the  term  "  synapsis," 
is  spoken  of  by  some  writers  as  "  syndesis,"  and  by  others  as  "conjugation."  The 
use  of  the  latter  term  seems  likely  to  lead  to  confusion. 


138        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

intelligible  by  a  careful  study  of  Fig.  68,  which  represents  in  a 
very  diagrammatic  manner  the  formation  of  the  polar  bodies  and 
the  distribution  of  the  maternal  and  paternal  chromosomes  in  the 
maturation  of  a  typical  animal  ovum. 

Periodic  reduction  of  the  number  of  chromosomes  is  clearly  a 
necessary  consequence  of  the  sexual  process,  for  a  doubling  of 


C.3. 


FIG.  68. — Diagram  of  the  Maturation  of  a  typical  Animal  Ovum,  showing 
the  Behaviour  of  the  Maternal  and  Paternal  Chromosomes  and  the 
Formation  of  Polar  Bodies.  (The  somatic  number  of  chromosomes  is 
supposed  to  be  four ;  the  maternal  chromosomes  are  shaded  and  the 
paternal  not,  and  the  differences  between  the  two  of  each  set  are 
indicated  by  their  shapes.) 

A.,  ordinary  somatic  mitosis  in  an  oogonium,  each  chromosome  split ;  B,  daughter 
oogonium;  C,  synapsis  in  primary  oocyte  ;^D,  reducing  division,  formation  of  first 
pol  ar  body ;  E ,  commencement  of  formation  of  second  (or  third)  polar  body  by  ordinary 
mitosis  and  of  division  of  the  first ;  F,  mature  ovum  with  three  polar  bodies. 

chr.,  chromosomes ;  c.s.,  centrosome ;  n.m.,  nuclear  membrane ;  p.b.  1 — 3,  polar  bodies. 

the  number  at  every  zygosis  or  conjugation  of  gametes  could  not 
go  on  indefinitely  without  some  such  compensation.  In  animals, 
as  we  have  seen,  the  reduction  takes  place  during  the  maturation 
of  the  germ  cells,  but  it  is  by  no  means  necessarily  associated  with 
this  process.  In  the  ferns,  where  alternating  sexual  and  asexual 
generations  are  represented  by  independent  and  well  developed 
organisms,  the  reduction  takes  place  in  the  process  of  spore- 
formation  by  the  sporophyte.  Hence  the  gametophyte  (pro- 
thallus)  to  which  the  spore  gives  rise  has  in  all  its  somatic  cells 


POLAE  BODIES  IN  PROTOZOA 


139 


Head 


m.p. 


nu. 


Tail 


v.m. 


the  reduced  number  of  chromosomes,  while  the  sporophyte  (fern 
plant  proper),  owing  to  the  fact  that  it  develops  from  a  fertilized 
ovum  and  receives  a  set  of  chromosomes  from  each  parent,  has  the 
full  number,  or,  perhaps  we  should  say  more  correctly,  the  double 
number. 

Even  amongst  the  Protozoa  the  phenomenon  of  nuclear  reduc- 
tion has  been  observed,  and  it  probably  occurs  wherever  the  sexual 

process  takes  place.  Thus, 
in  the^case  of  Copromonas, 
the  life  history  of  which  we 
have  already  dealt  with, 
the  nucleus  of  each  of  the 
similar  gametes,  previous 
to  uniting  with  its  mate  to 
form  the  zygote  nucleus, 
gives  off  two  "polar  bodies" 
which  undergo  degenera- 
tion in  the  cytoplasm  (Fig. 
37, 3, 4),  and  something  very 
similar  may  be  observed  in 
the  maturation  of  the  ovum 
of  Coccidium.  In  the  con- 
jugating  Paramoacium, 
again,  the  products  of  divi- 
sion of  the  micronucleus 
(Fig.  41,  B,  C)  which  undergo 
no  further  development  may 
be  regarded  as  polar  bodies. 
As  we  have  already  seen, 
the  mature  gametes  through- 
out both  the  animal  and 
vegetable  kingdoms  usually 
exhibit  a  very  strongly  marked  sexual  dimorphism,  which 
attains  its  fullest  expression  in  the  differentiation  into  active 
spermatozoon  and  passive  ovum  (Fig.  69). 

A  typical  spermatozoon,  as7  we  have  also  pointed  outx  closely 
resembles  a  flagellate  protozoon.  It  consists  of  a  "  head  "  and 
a  "  tail,"  connected  together  by  a  "  middle  piece  "  (m.p.).  The 
head  contains  the  nucleus  (nit.),  which  almost  entirely  fills  it, 
being  covered  with  only  a  very  thin  envelope  of  cytoplasm.  The 
middle  piece  contains  a  centrosome  (cs.).  The  tail,  or  flagellum,  is 


FIG.  69. — Diagram  of  typical  Sperma- 
tozoon and  Ovum,  the  former  much 
more  highly  magnified  than  the 
latter. 

ax.,  axial  filament;  e.g.,  chromatin  granules; 
cs.,  centrosome ;  cyt.,  cytoplasm ;  m.p., 
middle  piece;  n.m.,  nuclear  membrane; 
nu.,  nucleus;  nuls.,  nucleolus;  v.m., 
vitelline  membrane ;  y.g.,  yolk  granules. 


140       OUTLINES   OF   EVOLUTIONAKY  BIOLOGY 


ch. 


ch. 


the   active  locomotor   organ,  by   the  movements  of  which   the 
spermatozoon  swims  about ;  it  contains  an  axial  filament  (ax.) 

and  may  or  may  not  be 
provided  with  a  "lateral 
undulating  membrane. 
A  typical  ovum  is  a 
relatively  large,  spheri- 
cal cell  with  a  con- 
spicuous nucleus  (nu.) 
surrounded  by  a  large 
quantity  of  cytoplasm 
(cyt.),  and  the  whole 
enclosed  in  a  delicate 
vitelline  membrane 
(v.m.).  The  actual  size 


a.c. 


alb. 


v.m. 


FIG.   70. — Diagram   of    the    Structure   of    a 

Bird's  Egg. 
a.c.,    air    chamber;   alb.,     albumen;   ch.,    chalazse,     of     the     OVUm    depends 

twisted  cords  of  dense  albumen  which  serve  to 

keep  the    "  yolk  "    in    position  ;  g.d.,    germinal 

disk;    nu.,    nucleus;    sh.,    shell;   sh.m.,    shell 

membrane;  v.m.,  vitelline  membrane ;  yk.,  "yolk." 


/^n 
On 


NU 


amount  of  food  material 
(deutoplasm    or    yolk), 

which  is  stored  up  in  the  cytoplasm  in  the  form  of  granules  (y  .g.). 

In  Amphioxus  (Fig.  13,  1)  the  amount  of  food  material  is  very 

small,  and  the  egg  is  only  about  ^Jo*n 

inch  in  diameter. 

An  extreme  contrast  to  the  egg  of 

Amphioxus  is  seen  in  that  of  a  bird 

(Fig.  70),  where  the  amount  of  yolk 

is   enormously  large   and   the   active 

protoplasm  is  confined   to  a  minute 

"  germinal  disk  "  (g.d.),  containing  the 

nucleus  (nu.),  which  lies  within  the 

vitelline  membrane  (v.m.)  on  the  top    M0/ 

of  the  "yolk"  (yk.),  while  the  ovum 

proper  is  entirely  enclosed  in  accessory 

structures  —  the  "  white  "  or  albumen 

(alb.)  and  the  shell  (sh.),  with  its  lining 

membrane  (sh.m.). 

The  mammalian  ovum,  on  the  other 

hand,  is,  like  that  of  Amphioxus,  very 

minute,  in  the  rabbit  (Fig.  71)  again  only  about  ^th  inch  in 

diameter.      It"  is  enclosed  in  an   envelope  known   as  the  zona 

radiata  (Z),  which  lies  outside  the  vitelline  membrane,  and  it 


IG' 


"  Vertebrate       Embry- 
ology," after  Bischoif.) 

MO,  spermatozoa  which  have 
penetrated  the  zona  radiata  ; 
N,  nucleus;  NU,  nucleolus; 
Z,  zona  radiata. 


ATTEACTION   OF   GAMETES  141 

contains  very  little  deutoplasm.  This  is  correlated  with  the  fact 
that  it  develops  within  the  body  of  the  parent  at  the  expense  of 
food  material  derived  from  the  blood  of  the  latter.  There  is 
reason  "to  believe,  however,  as  we  shall  see  later  on,  that  the 
small  size  of  the  mammalian  egg  is  a  secondary  feature. 

The  plant  egg-cell  may  also  be  loaded  up  with  food  material, 
so  as  to  attain  a  large  size,  as  in  the  green  alga,  Chara, 
where  the  contrast  between  the  minute  flagellate  spermatozoon 
and  the  relatively  gigantic  ovum,  filled  in  this  case  with  starch 
grains,  is  very  striking.  In  the  higher  plants,  however,  where, 
as  in  the  case  of  the  Mammalia,  the  developing  embryo  is  not 
dependent  for  its  nutrition  upon  food  supplies  stored  in  the  egg- 
cell,  the  latter  remains  quite  small,  as,  for  example,  in  the  fern 
(Fig.  52)  and  the  flowering  plant  (Fig.  55,  e). 

According  to  some  authorities  one  of  the  most  important 
differences  between  ovum  and  spermatozoon  in  animals  lies  in  the 
fact  that  the  centrosome  of  the  former  disappears  finally  during 
the  process  of  maturation,  the  centrosome  of  the  zygote  being 
contributed  by  the  spermatozoon  alone.  In  view  of  the  fact, 
however,  that  a  definite  centrosome  is  not  usually  recognizable  at 
all  in  the  higher  plants  we  cannot  attribute  very  great  importance 
to  its  supposed  absence  in  the  animal  ovum,  and  we  shall  also  see 
presently  that  centrosomes  appear  in  developing  eggs  which  have 
not  been  fertilized  by  spermatozoa. 

We  have  already  had  occasion  to  refer  to  the  existence  of 
some  attractive  force  whereby  the  male  and  female  gametes 
are  brought  together  in  conjugation.  Many  observers  main- 
tain that  this  is  simply  a  case  of  positive  chemotaxis,  or 
the  chemical  stimulation  of  the  protoplasm  of  one  gamete  by 
a  specific  secretion  of  the  other  in  such  a  way  as  to  cause 
them  to  respond  by  approaching  one  another  (or  by  the  male 
.gamete  approaching  the  female). 

It  is  a  well  known  fact  that  certain  spermatozoa  are  attracted 
by  specific  chemical  substances.  Thus  the  free-swimming  sperma- 
tozoa of  ferns  and  mosses  are  attracted  by  weak  solutions  of 
malic  acid  and  cane  sugar  respectively,  and  those  of  Coccidium 
are  attracted  by  nuclear  matter  discharged  from  the  ovum  in  the 
process  of  maturation. 

There  can  be  no  doubt  that,  whether  the  attracting  substance 
be  secreted  by  the  germ  cells  themselves  or  by  some  other  part 
$  the  organism,  chemotaxis  sometimes  plays  a  very  important 


142        OUTLINES   OF   EVOLUTIONAEY   BIOLOGY 

part  in  bringing  the  gametes  together.  So  also,  of  course,  do 
many  other  factors  in  various  plants  and  animals,  but  we  must 
distinguish  between  factors  which  act  directly  upon  the  gametes, 
such  as  chemotaxis,  and  those  which  act  indirectly  through  the 
soma  or  body  of  the  organism,  as  for  example  through  the  visual 
and  olfactory  senses  of  the  higher  animals. 

It  is  probable,  however,  that  chemotaxis  itself  is  but  a  secondary 
factor  which  serves  to  bring  the  gametes  within  the  range  of  one 
another's  direct  influence.  Thus  in  Coccidium  (Fig.  39)  the 
chemotactic  action  seems  to  be  exhausted  after  a  certain  number 
of  spermatozoa  have  been  attracted  to  the  neighbourhood  of  the 
ovum  and  a  fresh  attraction  appears  to  be  exerted  by  the  ovum 
itself  or  by  its  nucleus. 

The  term  cytotropism,  or  cytotaxis,  has  been  applied  to  the 
attraction  which,  according  to  some  observers,  is  sometimes 
set  up  between  two  adjacent  cells,  and  something  of  this  kind 
probably  comes  into  play  in  the  mutual  attraction  of  gametes. 
It  can  probably  act  only  at  very  short  distances,  and  hence  the 
necessity  for  some  preliminary  means  of  attraction  such  as 
chemotaxis.  That  chemotaxis  alone  is  not  a  sufficient  explana- 
tion of  the  phenomenon  in  question  is  suggested  by  the  case  of 
Spirogyra.  The  conjugation  of  the  gametes  in  this  plant  has 
already  been  described  in  Chapter  VIII.  It  will  be  remembered 
that  the  process  may  take  place  between  the  cells  of  two  filaments, 
lying  close  together,  parallel  with  one  another  (Figs.  43  and 
44),  and  is  then  inaugurated  by  those  of  the  male  filament. 
Each  of  these  cells  which  happens  to  lie  opposite  to  a  cell  of  the 
female  filament  puts  forth  a  hollow  protuberance  of  its  wall,  which 
is  presently  met  by  a  similar  protuberance  from  the  wall  of  the 
female  cell,  the  two  projections  uniting  to  form  a  canal  through 
which  the  protoplasmic  body  of  the  male  gamete  creeps  inside  the 
cell- wall  of  the  female  gamete  to  conjugate  with  the  latter.  It 
sometimes  happens,  however,  that,  owing  to  inequalities  in  the 
sizes  of  the  cells,  there  may  be  a  cell  in  one  filament  which  lies 
between  two  cells  of  the  opposite  filament  and  for  which  there  is 
no  mate,  all  the  adjacent  cells  being  coupled.  In  such  cases  the 
solitary  cell  (Figs.  43  and  44,  S.C.),  if  it  exhibits  any  of  those  re- 
markable activities  which  are  shown  by  the  conjugating  cells  on 
either  side  of  it,  merely  makes  preliminary  advances  which  are 
prematurely  checked,  as  though  there  were  a  competition  for 
partners  in  which  it  was  unsuccessful.  Here  it  is  obvious  that 


PARTHENOGENESIS  143 

in  the  case  of  two  cells  lying  opposite  to  one  another,  though  not 
in  contact,  and  though  each  is  enclosed  in  a  firm  cell-wall,  some 
stimulus  is  transmitted  from  one  to  the  other  which  calls  forth 
a  definite  response  manifested  in  the  formation  of  the  connecting 
canals  and  the  conversion  of  the  protoplasmic  contents  of  the 
cells  into  gametes.  The  insufficiency  of  the  principle  of  chemo- 
taxis  to  account  for  these  phenomena  appears  to  be  indicated 
by  the  fact  that  cells  which  have  no  mates  do  not  form  either 
complete  connecting  canals  or  gametes,  though  apparently 
exposed  equally  with  their  neighbours  to  the  influence  of  any 
chemical  substances  dissolved  in  the  surrounding  water.  A 
possible  explanation  appears  to  be  that  the  solitary  cell  cannot 
attract,  or  at  any  rate  retain,  the  attention  of  a  mate  to  stimulate 
it  to  complete  the  process  of  conjugation.1 

The  cytotropic  attraction  of  the  gametes,  as  we  have  already 
observed,  probably  depends  upon  some  difference  of  polarity 
between  the  two.  That  it  is  mutual  is  demonstrated  by  such 
cases  as  that  of  Zygogonium  (see  p.  97)  and  by  the  fact  that  even 
when  the  ovum  is  too  heavily  laden  with  food  material  to  take  any 
active  part  in  the  process  of  conjugation  it  yet  in  many  cases  puts 
out  a  definite  "  cone  of  attraction  "  towards  the  advancing  sper- 
matozoon, as  seen  in  Coccidium  (Fig.  39).  What  the  real  nature 
of  this  primary  attraction  between  the  gametes  is  we  do  not  know ; 
it  may  ultimately  be  explicable  in  terms  of  some  force  already 
known  to  us,  or  it  may  be  one  of  those  cases  where  it  will  be 
convenient  to  cloak  our  ignorance  by  the  assumption  of  some 
special  vital  force  of  which  we  know  nothing. 

Although  as  a  general  rule  an  egg  does  not  develop  unless 
fertilized  by  a  spermatozoon,  this  is  by  no  means  always  the 
case,  and  many  instances  are  known  of  parthenogenesis  or  the 
development  of  unfertilized  eggs.  This  may  either  be  a  normal 
occurrence  in  the  life  cycle  or  it  may  be  artificially  induced. 

Natural  parthenogenesis  occurs  chiefly  in'  insects,  especially 
amongst  the  aphides  or  plant  lice.  In  these  animals  males  and 
perfect  females  appear  only  in  the  autumn.  Fertilized  eggs  are 
then  laid  which  hibernate  through  the  winter  and  hatch  in  the 
spring, producing  imperfect  viviparous  females.  In  these  imperfect 
females  eggs  are  formed  which  develop  parthenogenetically  within 
the  body  of  the  parent  and  give  rise  to  fresh  generations  of 
viviparous  forms.  This  reproduction  by  means  of  unfertilized  eggs 
*  See,  however,  the  footnote  on  p.  189. 


144        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

is  repeated  again  and  again  throughout  the  summer  months  and 
thus  the  aphides  multiply  with  great  rapidity.  In  the  autumn, 
however,  males  and  perfect  females  are  again  produced.  The 
viviparous  imperfect  females,  as  well  as  the  males,  are  generally 
winged,  the  perfect  females  are  wingless.  We  have  here  another 
kind  of  alternation  of  generations,  in  which  forms  which  repro- 
duce parthenogenetically  alternate  with  others  which  exhibit 
the  normal  sexual  process ;  to  this  type  of  alternation  the 
term  heterogeny  is  sometimes  applied.  A  very  large  number 
of  parthenogenetic  generations  may  intervene  between  two 
sexual  ones. 

Another  well-known  case  of  parthenogenesis  is  that  of  the  hive- 
bee,  where  the  eggs  laid  by  the  queen  may  either  be  fertilized  or 
not,  in  the  former  case  giving  rise  to  females  (workers  or  queens) 
and  in  the  latter  to  males  (drones).  Other  instances  occur  amongst 
those  parasitic  flat-worms  known  as  flukes  (Trematoda)  and  in 
some  Crustacea  (Cladocera,  the  so-called  water  fleas). 

The  most  remarkable  cases  of  natural  parthenogenesis,  how- 
ever, are  those  to  which  the  special  term  paedogenesis  has  been 
applied,  in  which  the  imperfect  females  do  not  even  wait  to 
attain  maturity  before  producing  their  offspring,  but  actually  do 
so  in  the  larval  condition,  as  in  Chironomus  and  some  other 
two-winged  flies  (Diptera). 

i  In  general  we  may  say  that  parthenogenesis  occurs  in  cases 
'where  it  is  desirable  to  take  advantage  of  a  brief  season  of  favour- 
able conditions  to  multiply  the  race  as  rapidly  as  possible.  It  is 
necessary  to  make  hay  while  the  sun  shines.  When  adverse 
conditions  set  in,  such  as  the  advent  of  winter  in  the  case  of  the 
aphides,  or  discharge  from  the  body  of  the  host  in  the  case  of 
parasitic  flukes,  the  vast  majority  of  the  race  will  perish,  but 
a  sufficient  number  will  be  able  to  protect  themselves  in  some 
way  (like  the  encysted  cercariae  of  the  fluke),  or  a  sufficient 
number  of  fertilized  and  protected  eggs  will  be  produced  (as  in 
the  aphides)  to  tide  over  the  evil  time  •  and  form  the  starting 
points  for  fresh  generations  at  the  first  favourable  opportunity. 

It  soems  possible,  also,  that  in  some  cases  parthenogenesis 
may  be  continued  indefinitely  without  fertilization  ever  occurring, 
for  in  certain  species  of  minute  rotifers  and  crustaceans  no  males 
have  as  yet  been  observed. 

Recent  researches  have  shown  that  parthenogenesis  can  be 
artificially  induced  in  cases  where  it  does  not  occur  naturally  at 


ABTIFICIAL   PARTHENOGENESIS  145 

all,  and  Professor  Loeb1  goes  so  far  as  to  maintain  that  the 
problem  of  fertilization  is  really  one  of  physical  chemistry.  He 
holds  that  the  development  of  the  egg  is  to  be  regarded  as  a 
chemical  process  which  depends  mainly  on  oxidation,  and  finds 
that  the  unfertilized  eggs  of  various  animals  (sea-urchins  and 
worms)  will  undergo  development  (at  any  rate  up  to  a  certain 
point)  after  exposure  to  the  action  of  certain  chemical  reagents. 
The  unfertilized  eggs  of  a  sea-urchin,  for  example,  developed  into 
larvae  after  being  placed  for  two  hours  in  sea  water,  the  osmotic 
pressure  of  which  had  been  raised  about  60  %  by  the  addition  of 
some  kind  of  salt  or  sugar,  but  this  "  hyper  tonic  "  solution  must 
contain  a  sufficient  quantity  of  free  oxygen.  In  another  case  the 
unfertilized  eggs  of  the  worm  Chaetopterus  were  stimulated  to 
develop  into  larvae  by  the  mere  addition  of  potash  and  acids, 
without  the  osmotic  pressure  of  the  sea  water  being  raised. 

Exactly  what  takes  place  under  these  circumstances  we  do  not 
know,  and  any  speculation  on  this  point  is  perhaps  somewhat 
premature,  but  it  is  quite  clear  from  the  experiments  of  Loeb  and 
other  workers  in  the  same  field  that  we  can  no  longer  regard 
fertilization  as  an  indispensable  condition  of  development  even  in 
the  case  of  eggs  which  do  not  naturally  exhibit  the  phenomenon 
of  parthenogenesis.  These  experiments  may  also  throw  some 
light  upon  the  process  of  normal  fertilization,  especially  as 
regards  the  nature  of  the  actual  stimulus  which  causes  the 
fertilized  egg  to  begin  segmenting. 

The  casting  out  of  polar  bodies  during  the  maturation  of  the 
ovum  led  many  of  the  earlier  observers  of  this  phenomenon  to 
believe  that  the  matured  ovum  is  incapable  of  development 
because  it  has  an  imperfect  nucleus,  the  importance  of  the 
nucleus  as  taking  the  lead  in  cell-division  having  been  established 
at  a  comparatively  early  date.  The  imperfect  nucleus  of  the 
matured  ovum  was  termed  the  female  pronucleus,  and  it  was 
supposed  to  be  converted  into  a  perfect  segmentation  nucleus  by 
union  with  the  male  pronucleus  brought  into  the  egg  by  the 
spermatozoon  (Fig.  64),  and  the  power  of  cell-division  was  supposed 
to  result  from  this  completion  of  the  nucleus.  The  observations 
upon  which  this  belief  was  based  were  perfectly  correct,  but  the 
conclusions  drawn  from  them  have  not  been  sustained  by  recent 
investigations,  for  in  cases  of  artificial  parthenogenesis  development 

1  "  Die  chemische  Entwicklungserregung  des  tierischen  Eies "  (kunstliche  Par« 
thenogenese).  Berlin,  1909. 

B.  L 


146        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

takes  place  in  spite  of  the  fact  that  redaction  of  the  nucleus  of 
the  ovum  has  occurred  in  maturation  and  has  not  been  made 
good  by  union  with  the  nucleus  of  a  spermatozoon. 
D  In  cases  of  parthenogenesis  it  is  clear  that  the  developing 
organism  is  provided  only  with  maternal  chromosomes,  but  we  are 
now  also  acquainted  with  cases  which  form  the  exact  converse  to 
this,  only  paternal  chromosomes  being  present.  Such  cases  may 
arise  when  an  ovum  is  artificially  enucleated,  so  that  all  the 
maternal  chromosomes  are  removed,  and  then  fertilized  by  a 
spermatozoon.  It  has  actually  been  found  possible  in  this  way  to 
induce  the  development  of  enucleated  eggs,  and  the  phenomenon, 
our  knowledge  of  which  we  owe  mainly  to  Delage,  is  known  as 
merogony.  Although  it  is  obvious  that,  as  a  result  either  of 
artificial  parthenogenesis  or  of  merogony,  the  developing  organism 
may  start  life  with  only  half  the  normal  number  of  chromosomes, 
it  is  possible  that,  as  maintained  by  Delage,  this  number  may  be 
subsequently  doubled  in  some  way. 

It  cannot  therefore  be  the  union  of  male  and  female  pronuclei 
that  furnishes  the  stimulus  to  development;  this  union,  or 
amphimixis  as  Weismann  terms  it,  has  another  significance,  and, 
as  we  shall  see  later  on,  is  most  probably  connected  with  the 
transmission  of  inherited  characters  from  parent  to  offspring. 

Boveri,  followed  by  other  observers,  has  put  forward  the  view 
that  the  unfertilized'egg  cannot  develop  because  the  centrosome, 
which  is  to  be  regarded  as  the  dynamical  centra  of  the  cell,  has 
been,  eliminated  during  the  process  of  maturation.  It  is  the 
spermatozoon  that,  in  the  act  of  fertilization,  brings  with  it 
the  centrosome  upon  the  activity  of  which  the  cell-divisions  of  the 
fertilized  egg  depend.  This  is  probably  perfectly  true  in  cases 
of  normal  fertilization  and  development  in  animals,  but  the  view 
that  the  centrosome  of  the  spermatozoon  supplies  the  essential 
stimulus  to  development  seems  to  be  hopelessly  negatived  by  the 
phenomena  of  parthenogenesis,  in  which  new  centrosomes 
undoubtedly  arise  in  unfertilized  eggs. 

It  seems  therefore  as  if  we  were  for  the  present  thrown  back 
upon  Loeb's  hypothesis  of  a  chemical  stimulus,  which  he  main- 
tains for  normal  fertilization  as  well  as  for  artificial  partheno- 
genesis. In  the  former  case  the  necessary  chemical  substances 
are  supposed  to  be  brought  in  by  the  spermatozoon.  Loeb 
believes  that  there  are  two  of  thess  substances.  One,  which  he 
terms  a  lysin,  is  supposed  to  bring  about  the  formation  on  the 


10 
STIMULUS   TO   DEVELOPMENT  l.' 

surface  of  the  egg  of  the  "  fertilization  membrane,"  while  the 

\  other  is  supposed  to  prevent  the  cytolysis  or  disintegration  of  the 

\  ovum,  induced  by  the  formation  of  the  fertilization  membrane, 

from  going  too  far.      It  remains  to  be  seen  whether  this  "  lysin 

^  theory  "  will  stand  the  test  of  time. 

vWhat  it  is  that  stimulates  the  unfertilized  ovum  to  develop  in 
normally  occurring  parthenogenesis  we  do  not  yet  know.  In 
most  cases  of  this  kind  the  process  of  maturation  seems  to  differ 
more  or  less  from  that  which  takes  place  in  eggs  which  are 
destined  to  be  fertilized  by  a  spermatozoon,  and  it  may  be  that 
these  differences  have  something  to  do  with  the  power  of  the  egg 
to  develop  parthenogenetically,  but  the  discussion  of  this  very 
difficult  problem  is  altogether  beyond  the  scope  of  the  present 
work. 


L  2 


PART  III.— VARIATION  AND  HEREDITY 
CHAPTEE  XI 

Variation — Meristic  and  substantive  variations — Fluctuations  and  muta- 
tions— Soinatogenic  and  blastogenic  variations — Origin  of  blastogenic 
variations. 

THE  term  variation  is  used  in  more  than  one  sense ;  it  may 
be  defined  in  the  first  instance  as  the  process  whereby  closely 
related  organisms  come  to  differ  amongst  themselves.  It  is  a 
matter  of  everyday  experience  that  neither  animals  nor  plants 
exhibit  absolutely  fixed  and  constant  characters,  which  are 
handed  on  without  alteration  from  parent  to  offspring.  This 
is  very  well  seen  in  the  case  of  human  families,  in  which 
there  is  rarely  any  difficulty  in  distinguishing  the  different 
members  by  more  or  less  pronounced  and  characteristic 
individual  traits.  One  may  be  fair  and  another  dark,  one  short 
and  another  tall,  one  with  brown  eyes  and  another  with  blue, 
one  clever  and  another  stupid,  and  so  on.  In  this  way  they  vary 
amongst  themselves  and  deviate  from  the  common  parents  of  the 
family  often  to  a  very  considerable  extent. 

We  cannot,  however,  avoid  extending  the  use  of  the  term 
variation  from  the  process  itself  to  the  results  of  that  process, 
and  speaking  of  organisms  as  exhibiting  variations,  but  this  usage 
is  not  likely  to  cause  any  confusion. 

Variations  are  of  many  kinds  and  may  be  classified  in  different 
ways  according  to  the  point  of  view  from  which  we  regard 
them. 

v  Meristic  variations,  which  are  variations  in  the  number  of  the 
repeated  parts  of  an  organism,  are  sometimes  contrasted1  with 
substantive  variations,  which  depend  upon  the  structure  (in- 
cluding shape,  size  and  colour)  of  the  organism  or  its  parts. 

Small  fluctuating  or  continuous  variations,   which  fluctuate 

1  Bateson,  "  Materials  for  the  Study  of  Variation."  (MacmiHan  &  Co.,  London 
1894.) 


1IEEISTIC  VARIATIONS  *  149 

'_> 

about  a  typical  condition,  are  contrasted  with  sudden,  discon- 
tinuous variations,  or  mutations. 

*"  Somatogenic  variations,  which  affect  the  body  of  the 
organism  and  are  acquired  in  the  life-time  of  the  individual,  are 
contrasted  with  blastogenic  or  germinal  variations,  which  arise  as 
a  consequence  of  some  modification  in  the  germ  cells. 

These  three  methods  of  classification  obviously  overlap  one 
another.  Thus  a  meristic  variation  may  be  continuous  or 
discontinuous,  somatogenic  or  blastogenic,  and  so  on,  but  it  will 
be  convenient  to  deal  with  each  group  separately. 
/  Meristic  or  Numerical  Variations.  In  a  very  great  number  of 
organisms  certain  parts  are  repeated  with  a  greater  or  less 
degree  of  regularity  and  constancy,  and  it  is  upon  this  repetition 
that  the  symmetry  of  the  organism  very  largely  de~pends7~  An 
ordinary  star-fish,  for  example,  is  radially  symmetrical,  with  five 
similar  arms  or  rays  radiating  from  a  common  centre ;  meristic 
variation,  however,  not  infrequently  gives  rise  to  six-raye'd 
individuals.  The  common  jelly-fish,  Aurelia  aurita,  again,  usually 
has  four  principal  radii,  but  specimens  are  occasionally  found  with 
two,  three,  or  six. 

All  vertebrate  animals,  on  the  other  hand,  and  many  inverte- 
brates, are  bilaterally  symmetrical,  and  at  the  same  time  meta- 
merically  segmented,  i.e.  with  repetition  of  similar  parts  in  linear 
series  one  behind  the  other.  The  vertebral  column,  for  example, 
is  made  up  of  a  larger  or  smaller  number  of  morphologically 
equivalent  units,  the  vertebrae,  to  which  the  ribs  are  attached  in 
linear  series  on  each  side,  and  the  limbs  are  arranged  in  two  pairs, 
in  each  of  which  the  same  fundamental  structure  can  be  traced. 
Many  instances  occur  in  different  animals  of  variation  in  the 
number  of  vertebrae,  and  in  man  the  occasional  occurrence  of 
thirteen  ribs  instead  of  the  normal  twelve  on  each  side  is  well 
known.  Cases  of  polydactylism,  or  the  development  of  extra 
digits  on  hand  or  foot,  also  come  under  the  head  of  meristic 
variation. 

In  the  vegetable  kingdom,  where  the  repetition  of  similar 
parts,  such  as  the  leaflets  of  leaves,  is  even  more  conspicuous 
than  amongst  animals,  meristic  variation  is  again  a  common 
phenomenon.  A  clover  leaf,  for  example,  normally  consists  of 
three  leaflets,  but  the  occasional  discovery  of  a  specimen  with  four 
has  been  a  source  of  satisfaction  to  superstitious  folk  from  time 
immemorial.  We  meet  with  similar  variations  in  the  number 


150       OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

of  petals  in  flowers  and  the  number  of  flowers  in  inflorescences. 
An  example  of  this  will  be  given  later  on  in  the  case  of  the 
grape  hyacinth. 

*S  Substantive  Variations.  Examples  of  this  type  of  variation  are 
seen  in  the  shape,  size  and  colour  of  the  human  body  and  of  its 
various  parts.  The  total  height  of  the  individual,  the  shape  of 
such  features  as  the  nose,  ears  and  fingers,  the  colour  of  eyes 
and  hair,  are  all  subject  to  very  great  variation.  The  same  is 
true  of  the  shape  and  size  of  leaves  and  the  colour  of  many 
flowers.  Numerous  physiological  variations,  which  doubtless 
depend  upon  unrecognized  and  perhaps  unrecognizable  differences 
in  structure,  may  also  be  classed  under  this  head,  as  for  example 
the  variations  in  the  egg-laying  powers  of  fowls  and  the  milk- 
giving  powers  of  cows,  and  in  the  percentage  of  sugar  present 
in  the  roots  of  the  sugar  beet. 

y  Fluctuating  Variations.  The  variations  which  come  under  this 
heading  are  also  sometimes  spoken  of  as  normal  or  continuous. 
They'  may  be  either  meristic  or  substantive.  They  are 
characterized  by  the  fact  that  they  pass  into  one  another  by 
small  or  even  insensible  gradations,  which  fluctuate  on 
either  side  of  a  typical  condition,  so  that  the  extent  to  which 
a  given  organism  (or  part  of  an  organism)  varies  can  be 
graphically  represented  in  the  form  of  a  curve.  Fluctuating 
meristic  variations  lend  themselves  most  readily  to  this  graphic 
method  of  treatment,  for  all  we  have  to  do  is  to  count  the 
numerically  varying  parts  in  a  sufficiently  large  number  of  cases 
and  plot  our  curve  accordingly. 

The  manner  in  which  this  is  done  is  represented  in  the 
accompanying  diagram  (Fig.  72),  which  expresses  graphically  the 
results  obtained  by  counting  the  number  of  flowers  in  202  in- 
florescences of  the  common  grape  hyacinth  (Muscari  sp.).  The 
number  of  flowers  in  an  inflorescence  ranged  from  20  to  42,  and 
the  number  of  inflorescences  showing  each  of  the  different  numbers 
of  flowers  ranged  from  1  to  29. 

The  base  line  of  the  diagram  is  divided  into  a  number  of  equal 
parts  (abscissae)  corresponding  to  the  number  of  flowers  (from  20 
to  42).  At  one  side  an  ordinate  is  erected  on  the  base  line  and 
divided  into  a  number  of  equal  parts  corresponding  to  the 
numbers  of  the  individual  inflorescences  (from  1  to  29)  bearing  the 
different  numbers  of  flowers.  Lines  are  then  ruled  parallel  to 
the  base  line  and  to  the  ordinate  respectively,  so  as  to  divide  the 


FLUCTUATING  VABIATIONS 


151 


paper  into  a  number  of  equal  squares  (or  the  proceeding  may  be 
simplified  by  working  with  already  squared  paper).  The  diagram 
is  completed  by  shading  in  an  appropriate  square  for  each 
inflorescence,  the  position  of  the  square  being  determined  by  the 
number  of  flowers  which  it  contains,  and  by  the  number  of 


20  21  22  25  24  75  26  27  28  29  30  31  52  35  34  35  36  57  38  39  40  41 

Number  of  flowers  in  each  Inflorescence 

EIG.  72.— Fluctuating  Variation  in  Inflorescences  of  the 
Grape  Hyacinth. 

similar  inflorescences  which  have  preceded  it  in  the  enumeration. 
Thus  an-  inflorescence  with  twenty-five  flowers  must  be  repre- 
sented by  a  square^  added  to  the  column  above  the  number  25 
on  the  base  line,  and  so  on.  If  now  we  join  the  middle  points  of 
the  tops  of  all  the  columns  by  straight  lines,  we  get  a  zig-zag  line 
which  represents  the  curve  of  variation  as  determined  by  our  actual 
observations.  It  is  obviously  a  very  imperfect  result,  but  its 


152        OUTLINES   OF   EVOLUTIONAKY  BIOLOGY 

imperfection  is  undoubtedly  due  in  the  main  to  the  fact  that  the 
number  of  observations  upon  which  it  is  based  has  not  been 
sufficiently  large  to  give  a  fair  average. 

The  larger  the  number  of  individual  cases  examined  and 
recorded  the  more  closely  will  our  zig-zag  line  approximate  to 
a  regular  curve.  Even  as  it  is,  with  all  its  imperfections,  it 
shows  certain  very  characteristic  features.  It  rises  steeply 
in  the  middle  and  falls  away  from  the  highest  point,  at  first 
suddenly  and  then  more  gradually,  on  either  side.  This  is  a 
graphic  representation  of  the  facts,  (1)  that  the  most  frequently 
occurring  number  of  flowers  in  the  inflorescence  (28)  lies  not  very 
far  from  midway  between  the  two  extremes,  and  (2)  that  the 


62    63    64    65    C6    67    68    69    70    71    72    73    74    75 


FIG.  73. — Fluctuating  Variation  in  Human  Stature.     (From  Lock.) 

more  the  number  of  flowers  deviates  from  this  on  either  side  the 
smaller  is  the  corresponding  number  of  inflorescences;  or,  to 
put  it  more  generally,  the  number  of  individuals  exhibiting  any 
given  degree  of  deviation  from  the  typical  condition  of  the 
species  is  inversely  proportional  to  the  amount  of  that 
deviation. 

Fig.  73  is  a  diagram  constructed  on  the  same  principles  as 
the  above,  but  based  upon  totally  different  material  and  a  very 
much  larger  number  of  observations.  It  is  taken  from  Mr.  R.  H. 
Lock's  work  on  "  Variation,  Heredity  and  Evolution,"  and  shows 
the  variation  in  stature  observed  amongst  4,42&  British  members 
of  the  University  of  Cambridge.  The  dots  in  this  diagram 
correspond  to  the  middle  points  of  the  tops  of  the  vertical  columns 
in  the  preceding  one.  The  statures  are  reckoned  in  the  nearest 


NORMAL   CURVE   OF  VARIATION  153 

whole  numbers  of  inches.  It  is  clear  that  a  zig-zag  line  formed 
by  joining  the  dots  in  the  diagram  would  approximate  fairly 
closely  to  the  curve  actually  drawn. 

The  normal  curve  of  variation  agrees  very  closely  with  what 
mathematicians  term  the  curve  of  frequency  of  error,  which  is 
the  graphic  representation  of  the  mode  of  occurrence  of  chance 
deviations  from  a  mean  or  average  and  may  be  derived  from  the 
theory  of  probability.  Such  a  curve  may  be  experimentally 
produced  by  drawing  a  vertical  line  on  a  target  and  firing  a  large 
number  of  shots  at  it.  There  will  be  a  more  or  less  strongly 
marked  tendency  for  the  shots  to  hit  the  line,  depending  upon 
the  skill  or  otherwise  of  the  marksman.  Most  of  them  will 
probably  strike  to  the  right  or  left  of  the  line  and  fairly  near  it, 
but  a  few  will  probably  be  very  wide  of  the  mark  on  either  side. 
If  the  distances  of  the  striking  places  from  the  vertical  line  be 
measured  an4  tabulated  the  result  may  be  expressed  in  the  form 
of  a  curve  which,  if  the  number  of  shots  be  large  enough,  will 
closely  resemble  the  curves  described  above.  The  fact  that  the 
normal  curve  of  fluctuating  variation  for  any  kind  of  organism 
is  practically  identical  with  the  mathematical  curve  of  frequency 
of  error  suggests  very  forcibly  that  the  variation  in  question  is 
due  to  chance  or  accident  causing  each  individual  in  the  course 
of  its  development  to  depart  more  or  less  from  the  typical 
condition  of  the  species  to  which  it- belongs.  These  deviations 
must  depend  upon  numerous  factors.  They  are  to  some  extent, 
no  doubt,  due  to  the  direct  influence  of  the  environment,  such 
as  the  effect  of  nutrition  upon  the  size  of  the  organism,  but 
they  may  also  depend  largely  upon  the  varying  characters  of  the 
germ  cells  from  which  the  organism  develops,  and  especially 
upon  the  permutations  and  combinations  of  characters  which 
happen  to  take  place  in  the  maturation  of  the  germ  cells  and  in 
their  sexual  union. 

In  short,  the  general  tendency  is  doubtless  for  each  individual 
to  conform  to  the  type  of  the  species  to  which  it  belongs,  but 
many  accidental  circumstances  combine  to  prevent  the  realization 
of  this  tendency  and  deviations  from  the  type  take  place  in 
accordance  with  the  laws  of  chance.  It  should  be  noted,  however,  j 
that  the  curve  of  variation  is  by  no  means  always  symmetrical. 

Mutations.  The  term  mutation,  or  discontinuous  variation,  is 
applied  to  the  process  whereby  new  and  more  or  less  conspicuous 
characters  appear  suddenly  and  spontaneously,  without  any 


154       OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

obvious  reason.  In  extreme  cases  the  organisms  exhibiting  such 
mutation  are  often  spoken  of  as  sports  or  monstrosities ;  the 
latter  term,  however,  is  also  frequently  applied  in  the  case  of 
purely  artificial  modifications,  which  must  not  be  included  under 
this  heading.  Such  artificial  modifications  are  not,  at  any  rate 
as  a  rule,  inherited  by  future  generations,  while  true  mutations 
usually,  if  not  invariably,  are. 

Mutations  differ  from  fluctuating  variations  not  only  in  that  they 
usually  deviate  more  widely  from  the  type  of  the  species,  but 
also  in  the  much  greater  rarity  of  their  occurrence  and  in  the 
fact  that  they  do  not  fluctuate  about  the  old  mean  or  average 
condition ;  nevertheless  it  may  be  questioned  whether  we  can 
logically  draw  a  hard  and  fast  distinction  between  the  two. 

Mutations  may  be  either  meristic  or  substantive.  Human 
beings  with  six  fingers  or  toes,  in  place  of  the  normal  five, 
are  not  infrequently  met  with,  and  this  meristic  mutation  (hexa- 
dactylism)  is  well  known  to  be  transmitted  by  heredity. 

The  classical  instance  of  the  Ancon  or  otter  sheep,  on  the 
other  hand,  affords  one  of  the  best  known  examples  of  substantive 
mutation.  To  quote  the  words  of  Huxley,1  "  It  appears  that  one 
Seth  Wright,  the  proprietor  of  a  farm  on  the  banks  of  the  Charles 
Kiver,  in  Massachusetts,  possessed  a  flock  of  fifteen  ewes  and  a  ram 
of  the  ordinary  kind.  In  the  year  1791,  one  of  the  ewes  pre- 
santed  her  owner  with  a  male  lamb,  differing,  for  no  assignable 
reason,  from  its  parents  by  a  proportionally  long  body  and 
short  bandy  legs,  whence  it  was  unable  to  emulate  its  relatives 
in  those  sportive  leaps  over  the  neighbours'  fences,  in  which  they 
were  in  the  habit  of  indulging,  much  to  the  good  farmer's  vexa- 
tion." The  inheritance  of  this  peculiarity  was  so  strong  that 
this  single  individual  actually  became  the  starting  point  of  a  new 
breed. 

Mutations,  more  or  less  pronounced  in  character,  are  also  not 
infrequently  met  with  in  the  vegetable  kingdom.  Foxgloves,  for 
example,  are  sometimes  found  in  which  some  or  all  of  the  petals 
are  converted  into  stamens,  and  it  has  been  proved  by  experi- 
ment that  this  peculiarity  is  handed  down  from  parent  to 
offspring. 

Professor  Hugo  de  Vries  has,  as  is  well  known,  made  a  special 
study  of  mutation  amongst  plants.  In  the  year  1886  this  dis- 
guished  botanist  found  a  large  number  of  specimens  of  the  evening 

4  "Darwiniana,"  p.  35. 


MUTATIONS 


155 


primrose,  (Enothera  lamarckiana,  growing  in  a  field  near  Amster- 
dam, whither  they  had  made  their  way  from  a  neighbouring 
garden.  The  plants  were  in  a  state  of  intense  variability  and  their 
seeds  gave  rise  to  several  quite  distinct  new  forms,  which,  if  they 
had  occurred  in  a  state  of  nature,  would  have  been  considered 
as  separate  species.1  Professor  De  Vries  attributes  great  import- 
ance to  mutations  as  the  starting  points  of  new  species,  which  he 
believes  to  arise  in  this  sudden  manner  rather  than  by  fluctuating 
variation.2  We  shall  discuss  this  point  in  a  subsequent  chapter. 
The  difference  between  fluctuating  variation  and  mutation  is 
sometimes  illustrated  by  means  of  the  model  known  as  Galton's 
polygon  (Fig.  74).  A  thick  slab  of  wood  is  cut  into  the  form  of 
a  polygon,  with  unequal  sides  and  capable  of  resting  in  a  position 
of  more  or  less  stable  equilibrium  upon  any  of  its  edges,  the 


I 


FIG.  74. — Model  of  a  Polygon  in  two  Positions,  illustrating  the  Difference 
between  Fluctuating  Variation  and  Mutation. 

degree  of  stability  depending  upon  the  position  of  the  centre  of 
gravity  above  the  edge  upon  which  it  rests. 

The  polygon  may  be  supposed  to  represent  an  organism,  or 
rather  a  number  of  successive  generations  of  an  organism, 
whose  stability  (or  adherence  to  type)  tends  to  be  more  or  less 
disturbed  by  the  unknown  factors  which  cause  variation.  If  the 
model  be  pushed  it  may  be  made  to  rock  backwards  and  forwards 
on  either  side  of  a  mean  or  average  position,  and  if  the  oscillation 
does  not  exceed  certain  limits  it  will  return  to  rest  in  that  position. 
This  oscillation  may  be  compared  to  fluctuating  variation.  If  the 
disturbing  force  be  sufficiently  great,  however,  the  model  will  topple 
over  into  a  new  position  of  stability  and  come  to  rest  on  another 

1  Considerable  doubt   exists,  however,  as  to  the  origin  of  these   "  mutations," 
which  may  conceivably  be  due  to  the  splitting  up  of  some  unknown  hybrid,  such 
as  is  known  to  take  placs  in  other  cases  (cf.  Chapter  XIV   and  p.  413). 

2  "The  Mutation  Theory."    Trans,  by  Farmer  and  Darbishire.     London,  1910. 


156        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

edge.  This  may  be  compared  to  the  process  of  mutation  or  discon- 
tinuous variation,  whereby  an  organism  acquires  a  new  type  of 
structure. 

Concerning  the  forces  which  bring  about  mutation  in  organisms 
we  know  little  or  nothing.  It  is  possible  that  in  some  cases  the 
mutation  may  be  gradually  prepared  within  the  germ  cells  long  \ 
before  it  manifests  itself — the  final  upsetting  of  the  equilibrium 
only  taking  place  on  the  addition  of  the  last  straw.  If  such  be 
the  case  there  is  no  need  to  suppose  that  mutations  differ 
essentially  in  nature  from  small,  fluctuating  variations.  Tower's 
observations  on  the  artificial  production  of  mutations  in  certain 
beetles,  however,  indicate  very  clearly  that  such  modifications 
may  apparently  arise  quite  suddenly  as  the  rgfptt  ol^o'ine  change 
in  the  environment  acting  directly  upon  *me  germ  cells  of  the 
parent.  These  observations  will  be  referred  to  again  at  the  close 
of  the  present  chapter. 

(  Somatogenic  Variations.  Characters  are  said  to  be  s0matogenic 
or  "  acquired  "  when  they  arise  in  the  life-time  of  the  individual 
exhibiting  them  and  owe  their  origin  to  the  direct  influence  of 
the  environment  upon  the  soma  or  body.  They  are,  usually  at 
any  rate,  not  transmitted  by  heredity  to  succeeding  generations, 
except  perhaps  to  a  very  limited  and  inappreciable  extent.1 
Under  this  heading  are  included  the  effects  of  use  and  disuse 
of  organs,  and  numerous  cases  in  which  modifications  of  the 
body  are  artificially  produced,  as  well  as  those  in  which  they  are 
due  to  natural  causes. 

Amongst  the  effects  of  use  and  disuse  we  may  mention,  on  the 
one  hand,  the  enlargement  or  the  atrophy  of  special  organs  con- 
sequent upon  the  extent  to  which  they  are  employed,  and,  on  the 
other,  the  effects  of  education.  The  muscles  of  an  athlete  may 
be  greatly  increased  in  size  by  constant  use,  and  similarly  if  one 
of  the  two  kidneys  be  removed  the  other,  having  more  work 
thrown  upon  it,  becomes  enormously  enlarged  ;  but  we  should  not 
expect  these  modifications  to  be  handed  on  to  the  next  generation. 
Children  have  been  taught  to  speak  ever  since  man  first  became 
differentiated  from  his  speechless  ancestors,  but  every  child  has  to 
learn  the  art  anew  and  if  brought  up  amongst  foreigners  will 
come  to  speak  a  language  different  from  that  of  its  parents. 

The  small  feet  of  Chinese  ladies  and  the  slender  waists  of 
many  Europeans  are  artificially  produced  somatic  modifications 

1  This  may  be,  however,  a  very  important  exception. 


ARTIFICIAL    MONSTROSITIES  157 

ivhich  are  well  known  not  to  be  inherited.     They  are  merely 
listortions,  effected  by  easily  recognizable  mechanical  agencies. 

Much  more  remarkable  and  difficult  to  understand  are  those 
3ases  in  which  the  addition  of  specific  chemical  reagents  to  the 
water  in  which  aquatic  larvae  are  developing  produces  such 
lefinite  and  extensive  modifications  of  structure  as  to  give  rise  to 
veritable  monstrosities.  The  " Lithium  larvae"  of  sea-urchins  and 
frogs  have  long  been  known  and  more  recently  Stockard  has 
described1  the  "Magnesium  larva"  of  the  fish  Fundulus  hetero- 
clitus  (Fig.  75).  He  found  that,  when  the  developing  embryos 
of  this  fish  are  subjected  to  the  influence  of  magnesium  salts 
dissolved  in  sea  water,  a  large  percentage  of  them  acquire  a 
*  cyclopean  "  character,  with  a  single  median  eye  in  place  of  the 
ordinary  pair.  Such  embryos  may  hatch  and  swim  about  in  a 
perfectly  normal  manner,"  but  it  is  not  known  whether  they  can 
be  reared  to  the  adult  condition.  These  observations  seem  to 
indicate  that  the  cyclopean  monsters  which  sometimes  occur  in 
man  and  other  mammals  may  also  be  somatogenic  variations  due 
bo  some  unknown  environmental  influence.  * 

Blastogenic  or  Germinal  Variations.  In  this  category  are  included 
those  variations  which  are  believed  to  owe  their  origin  to  some 
modification  in  the  germ  cells  from  which  the  organism 
exhibiting  them  has  developed. 

It  is  important  to  observe  that  the  term  congenital,  sometimes 
used  in  this  connection,  is  not  synonymous  with  blastogenic,  for  it 
is  obvious  that  animals  which,  like  the  mammalia,  remain  within 
the  womb  of  the  mother  during  the  early  stages  of  development, 
may  come  to  develop  purely  somatogenic  characters  before  birth, 
due  to  environmental  influences  acting  in  utero  (e.g.  poisoning  of 
the  foetus  due  to  parental  alcoholism). 

It  is  very  doubtful,  as  we  have  already  said,  whether  we 
can  really  draw  any  absolute  distinction  between  blastogenic 
and  somatogenic  characters, ^and  it  seems  by  no  means  impos- 
sible that  somatogenic  modifications  may  sooner  or  later 
make  an  impression  upon  the  germ  cells  and  thus  ultimately 
become  blastogenic.  This  point,  however,  will  be  discussed 
later  on. 

Blastogenic  modifications  are  from  their  very  nature  as 
attributes  of  the  germ  cells  handed  on  by  heredity  from  gene- 
ration to  generation.  All  true  mutations  must  be  regarded  as 

1  "Journal  of  Experimental  Zoology,"  February,  1909. 


158       OUTLINES  OF  EVOLUTIONARY  BIOLOGY 


B. 


E. 

FIG.  15. — Free-swimming  Larvoe  of  Fandulue  heteroditus.     (From  Stockard.) 

A.  Normal  larva,  with  anteriorly  placed  mouth  (M). 

B.  Incompletely  cyclopean  larva,  with  the  two  eyes  joined  and  occupying  the  position 

usually  taken  by  the  mouth. 

C.  Completely  cyclopean  larva,  with  single  antero-median  eye.     Dorsal  aspect. 

D.  Lateral  aspect  of  same,  showing  the  ventral  niouth  (M). 

E.  Ventral  aspect  of  same;   ys,  yolk  sac. 


TOWER'S  EXPERIMENTS  ON   BEETLES  159 

blastogenic  and  many  so-called  fluctuating  variations  may  also 
perhaps  belong  to  the  same  category. 

The  modifications  of  the  germ  cells  by  virtue  of  which  the 
offspring  come  to  differ  to  a  greater  or  less  extent  from  their 
parents  are,  as  we  have  seen,  often  attributed  in  large 
measure  to  permutations  and  combinations  of  different  characters 
which  take  place  in  the  sexual  process  (amphimixis)  and  the 
preceding  nuclearreduction.  It  has  long  been  suspected,  how- 
ever, that  thegerm  cells  themselves,  apparently  independently  of 
the  body  in  which  they  are  enclosed,  may  be  influenced  by  the 
environment  to  which  arc-animal  or  plant  is^  exposed,  and  the 
observations  of  Tower4  upon  beetles  'of  the  genus  Leptinotarsa 
may  be  referred  to  in  this 'connect ion. 

This  observer  considers  that  all  permanent  variations  in  these 
beetles,  so  far  as  can  be  discovered,  arise  in  the  germ  cells  and 
are  in  no  wise  the  results  of  inherited  somatic  modifications.  He 
attributes  their  appearance  to  the  direct  action  of  the  environment 
upon  the  germ  plasm  and  supports  his  views  by  a  series  of  very 
interesting  experiments.  He  subjected  the  parents  to  environ- 
mental stimuli  of  various  kinds  during  the  growth  and  matura- 
tion of  their  germ  cells,  and  then,  after  the  ova  had  been  fertilized, 
allowed  the  development  of  the  young  to  take  place  under  normal 
conditions.  The  parents,  having  already  reached  their  final  state, 
were  not  themselves  visibly  affected  by  the  stimuli,  but  a  large 
percentage  of  the  offspring  showed  surprising  modifications 
which  were  strictly  inherited.  These  modifications  appear  to  be 
in  no  way  adaptive.  They  seem  to  bear  no  relation  to  the  nature 
of  the  stimulus  which  calls  them  forth  and  to  be  of  no  value  to 
the  organism  in  the  struggle  for  existence. 

We  may  cite  one  example  of  Tower's  experiments.  Four  males 
and  four  females  of  Leptinotarsa  decemlineata  (the  potato  beetle) 
were  exposed  during  the  earlier  part  of  the  laying  period  (the 
eggs  being  matured  and  laid  in  successive  batches)  to  extremely 
hot,  dry  conditions  accompanied  by  low  atmospheric  pressure. 
The  eggs  were  removed  as  soon  as  laid  and  reared  under  natural 
conditions.  From  506  larvae  thus  reared  96  adult  beetles  were 
obtained,  of  which  82  were  of  a  form  known  as  pallida,  2  of  a  form 
known  as  immaculothorax,  and  the  remainder  unmodified.  During 

1  "An  Investigation  of  Evolution  in  Chrysomelid  Beetles  of  the  Genus  Leptino- 
tarsa," by  William  Laurence  Tower.  (Publications  of  the  Carnegie  Institution, 
Washington,  1906J 


160        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

the  later  part  of  the  laying  period  the  same  parents  were  kept 
under  normal  conditions  and  yielded  319  eggs  from  which  61 
normal  beetles  were  obtained  and  none  of  the  other  forms,  and 
these  normal  beetles  continued  to  breed  true  for  three  generations, 
after  which  they  were  killed. 

The  two  specimens  of  the  immaculothorax  form  obtained  in 
the  earlier  part  of  the  experiment  unfortunately  died  from 
disease,  as  also  did  all  but  two  of  the  pallida.  The  remaining 
two,  however,  both  being  male,  were  crossed  with  normal 
females,  yielding  hybrid  offspring  with  the  decemlineata  charac- 
ters dominant,  and  these  hybrids,  breeding  inter  se,  gave  off- 
spring which  separated  out  in  a  characteristic  Mendelian  fashion1 
into  pallida,  decemlineata  and  hybrids  again.  There  can  be  no 
question  therefore  that  the  pallida  characters,  first  due  to  modi- 
fication of  the  germ  cells  by  the  action  of  changed  environment, 
were  strictly  heritable. 

It  should  be  observed  that  the  forms  pallida  and  immaculothorax 
also  occur  occasionally,  but  rarely,  in  a  state  of  nature  as  sports 
or  mutations,  a  fact  which  suggests  that  sports  or  mutations  in 
general  may  owe  their  existence  to  the  apparently  direct  action 
of  the  environment  upon  the  germ  cells.  It  is,  of  course,  possible, 
or  even  probable,  that  the  change  in  the  environment  merely  acts 
as  a  kind  of  liberating  stimulus,  which  enables  characters  already 
latent  in  the  germ  cells  to  express  themselves  in  the  developing 
organism,  which,  under  normal  conditions,  they  are  unable  to  do. 

We  shall  have  to  return  to  the  question  of  the  origin  of  blasto- 
genic  variations  in  future  chapters. 

i  See  Chapter  XIV. 


CHAPTER  XII 

Heredity  —  General  observations  —  Darwin's  theory  of  pangenesis  and 
Weismann's  theory  of  the  continuity  of  the  germ  plasm — The  nucleus 
as  the  bearer  of  inheritable  characters. 

WHEN  we  study  the  life  histories  of  the  unicellular  Protista 
we  find  ourselves  face  to  face  with  the  problem  of  heredity  in 
its  simplest  form.  The  Amoeba  divides  into  two  parts  by  simple 
fission,  the  division  of  the  cell  body  being  preceded  by  that  of 
the  nucleus.  The  two  daughter  cells  exactly  resemble  one 
another,  and,  except  in  point  of  size,  also  resemble  the  parent, 
while  the  latter  ceases  to  exist  as  an  individual  in  the  very  act  of 
reproduction.  Here  we  may  suppose  that  we  are  dealing  with  a 
division  which  is  qualitative  as  well  as  quantitative,  that  every 
organ  possessed  by  the  parent  cell  is  divided  into  two  similar 
parts  and  the  total  inheritance  thus  fairly  apportioned  between 
the  offspring,  which  will  therefore  be  exactly  similar  to  one 
another  and  will  need  only  to  feed  and  grow  in  order  to  become 
exactly  similar  to  the  parent. 

The  young  Amoebae  may  be  supposed  to  resemble  the  parent 
because  they  arise  by  duplication  of  the  parent  and  of  its  organs  j1 
moreover,  there  is  a  perfect  continuity  of  the  living  substance,  or 
protoplasm,  of  which  the  body  is  composed  from  one  generation 
to  the  next,  and  the  whole  of  the  body  of  the  parent  is  used  up 
in  providing  the  bodies  of  the  offspring.  Thus,  although  the 
individuality  oflthe  parent  comes  to  an  end,  the  Amoeba  never 
dies,  for  there  is  never  anything  left  over  to  die,  and,  barring 
accident,  it  goes  on  multiplying  for  ever.  We  have  here  a 
typical  illustration  of  the  so-called  immortality  of  the  Protista. 

The  case  is  very  different  amongst  the  higher  plants  and 
animals.  Here,  as  we  have  already  seen,  each  individual  starts 

1  We  cannot,  however,  say  this  of  all  Protista,  for  in  many  cases  the  division 
takes  place  asymmetrically  and  entirely  new  organs  have  to  be  formed  by  one  or  both 
of  the  daughter  cells,  as,  for  example,  in  the  transverse  fission  of  Bodo  (Fig.  38,  D — F). 
In  such  cases  it  looks  as  if  the  nucleus  might  be  the  real  seat  of  the  inherited 
tendencies,  and  as  if  it  were  able,  by  its  influence,  to  mould  the  daughter  cell  into 
the  form  of  the  parent. 


162       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

its  life  as  a  single  cell— the  fertilized  ovum  or  zygote— which  is 
strictly  comparable  to  a  unicellular  protistan.  Like  the  Amoeba 
it  divides  (under  favourable  circumstances)  repeatedly,  but  the 
products  of  division,  instead  of  separating  from  one  another  and 
going  each  its  own  way  as  an  independent,  unicellular  organism, 
all  remain  together  and  co-operate  with  one  another  to  form  a 
multicellular  body  of  greater  or  less"  complexity .  The  cells  of 
which  this  body  is  composed  become  differentiated  and  specialized 
in  various  directions.  In  so  doing  the  vast  majority  of  them 
lose  their  faculty  for  independent  existence,  and  when  their  powers 
of  division  have  become  exhausted  the  tissues  into  which  they 
are  combined  become  worn'  out  and  ultimately  die.  Thus  the 
body  or  soma,  as  a  whole,  must  suffer  death  sooner  or  later.  The 
only  cells  which  are,  even  potentially,  exempt  from  this  fate  are 
the  germ  cells.  These,  instead  of  becoming  highly  specialized 
constituents  of  the  soma,  remain  in  the  condition  of  more  or  less 
independent  Protista,  and  have  the  power  of  separating  sooner 
or  later  from  the  parent  body.  >  Like  most  of  the  Protista 
they  have  also  the  habit  of  conjugating  in  pairs  and  thereby 
renewing  their  powers  of  cell-division,  and-  thus  arise  the  zygotes 
or  fertilized  eggs  from  which  the  newv individuals  take  their 
origin. 

The  eggs  from  which  the  individuals  of  different  kinds  of 
plants  and  animals  arise  are  for  the  most  part  extraordinarily 
similar  to  one  another.  The  ovum  of  a  rabbit,  as  we  have 
already  seen,  is  a  minute  nucleated  mass  of  protoplasm  about 
^Jo^h  inch  in  diameter,  that  of  a  human  being  is  a  similar  but 
somewhat  larger  cell,  and  if  the  ova  of  a  hundred  different  kinds 
of  mammals  were  mixed  together  it  would  be  an  extremely 
difficult,  if  not  impossible  task  to  sort  them  all  out,  even  after 
the  most  minute  microscopical  examination.  Even  where  con^. 
spicuous  differences  exist,  as  between  the  eggs  of  a  mammal  and 
those  of  a  bird,  these  are  due  almost  entirely  to  the  development 
of  accessory  features,  such  as  protective  envelopes  and  food-yolk, 
which  have  little  to  do  with  the  nucleated  mass  of  protoplasm 
which  constitutes  the  really  vital  part  of  the  egg.  Yet  each  kind 
of  fertilized  egg,  if  it  develop,  will  give  rise  to  an  organism 
resembling  the  parent  from  which  it  was  itself  derived.  More- 
over, the  resemblance  will  not  be  merely  a  general  one,  it  will  be 
specific,  and  probably  even  more  than  specific,  for  it  may  include 
minute  individual  characters  peculiar  to  one  or  other  of  the 


PRE-FORMATION  AND  EPIGENESIS  163 

parents.  Everyone  is  familiar  with  cases  of  this  kind.  It  may 
be  some  abnormality  of  fingers  or  toes,  or  a  lock  of  white  hair  in 
some  special  situation  in  a  dark-haired  man,  or  even  some 
trifling  nervous  habit,  that  is  thus  indelibly  impressed  upon  the 
organism  and  handed  on  from  one  generation  to  another. 

Inasmuch  as  the  only  possible  connection  between  parent  and 
offspring  is  (in  most  cases)  through  the  germ  cells,  it  follows  that 
there  must  be  something  in  these  germ  cells  which,  so  to  speak, 
represents  all  the  inheritable  characters  of  the  parents  and  is 
capable  of  giving  rise  to  a  repetition  of  these  characters  in  the 
course  of  individual  development. 

Two  sharply  contrasted  views  as  to  what  takes  place  in  the 
development  of  the  egg  were  maintained  by  the  older  embryo- 
logists,  and,  in  a  modified  form,  survive  to  the  present  day.  The 
so-called  "evolutionists,"  or  " pre-formationists,"  maintained 
that  the  egg  contains  in  itself  a  complete  miniature  of  the 
organism  into  which  it  develops,  and  that  the  process  of 
development  consists  simply  in  an  unfolding  ("  evolution  ")  and 
growth  of  this  miniature.  This  idea,  of  course,  carried  to  its 
logical  conclusion,  involves  the  further  supposition  that  every 
egg  contains  in  miniature  the  bodies  of  all  future  generations, 
like  nests  of  boxes  one  within  the  other. 

In  opposition  to  this  view  the  upholders  of  the  theory  of 
"gpjgenesis  "  maintained  that  there  is  no  pre-formation  of  organs 
in  the  egg  but  that  the  different  parts  of  the  adult  organism 
become  gradually  differentiated  from  the  simple  undifferentiated 
ovum  during  the  course  of  development.  This  view,  which  is 
said  to  have  originated  with  Aristotle  and  was  strongly  supported 
by  the  great  pioneer  embryologist  C.  F.  Wolff  about  the  middle 
of  the  eighteenth  century,  almost  entirely  superseded  the  crude 
ideas  of  the  pre-formationists,  but  at  the  present  day  the  latter 
are  being  revived  to  some  extent,  but  in  a  more  refined  form,  as  a 
result  of  modern  experiments  in  embryology.  It  cannot  be 
disputed  that  in  some  cases  certain  parts  of  the  adult  organism 
can  be  traced  back  to  corresponding  portions  of  the  egg,  which 
cannot  therefore  be  entirely  undifferentiated,  and  it  is  probable 
that  in  the  end  the  truth  will  be  found,  as  in  so  many  other 
cases,  to  lie  in  a  compromise  between  the  two  extreme  views. 

The  great  problem  which  has  to  be  solved  by  any  theory  of 
heredity  is — How  do  the  apparently  simple  germ  cells  of  a  multi- 
cellular  organism  corne  to  be  representative  of  all  the  other 

M  2 


OUTLIN 


164       OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

cells  of  the  body,  so  that  when  they  develop  they  will  give 
rise  to  all  those  different  kinds  of  cells  arranged  in  the  same 
way  as  in  the  parent  ?  We  must  now  briefly  consider  some 
of  the  numerous  attempts  which  have  been  made  to  answer 
this  question. 

Darwin,  in  1868,  put  forward  his  theory  of  Pangenesis  as  a 
provisional  hypothesis  to  explain  the  facts  of  heredity,  and  this 
theory,  though  it  seems  never  to  have  met  with  any  large 
measure  of  acceptance,  is  of  considerable  historical  interest.1 
He  supposed  that  all  the  constituent  cells  of  which  the  body  is 
composed  not  only  multiply  by  ordinary  cell-division,  so  as  to 
build  up  the  various  tissues,  but  also,  throughout  life,  give  off 
extremely  minute  "  gemmutes  "  which  wander  through  the  body 
and  are  collected  in  vast  numbers  in  the  germ  cells.  The 
gemmules,  although  so  small  as  to  be  invisible  even  with  the 
highest  powers  of  the  microscope,  are  supposed  to  be  capable  of 
absorbing  nutriment  and  multiplying  by  division,  and  each  one 
is  supposed,  in  some  mysterious  and  unexplained  manner,  to 
represent  the  particular  cell  of  the  body  from  which  it  was 
derived,  and  to  be  capable,  at  the  proper  time  and  in  the  proper 
place,  of  impressing  the  character  of  its  parent  cell  upon  a 
corresponding  cell  of  the  new  organism  which  develops  from  the 
germ  cell. 

In  Darwin's  own  words : — 

"The  development  of  each  being,  including  all  the  forms  of 
metamorphosis  and  metagenesis,  depends  on  the  presence  of 
gemmules  thrown  off  at  each  period  of  life,  and  on  their  develop- 
ment, at  a  corresponding  period,  in  union  with  preceding  cells. 
Such  cells  may  be  said  to  be  fertilized  by  the  gemmules  which 
come  next  in  due  order  of  development.  Thus  the  act  of 
ordinary  impregnation  and  the  development  of  each  part  in  each 
being  are  closely  analogous  processes.  The  child,  strictly 
speaking,  does  not  grow  into  the  man,  but  includes  germs  which 
slowly  and  successively  become  developed  and  form  the  man.  In 
the  child,  as  well  as  in  the  adult,  each  part  generates  the  same 
part.  Inheritance  must  be  looked  upon  as  merely  a  form  of, 
growth,  like  the  self-division  of  a  lowly  organized  unicellular 
organism.  Reversion  depends  on  the  transmission  from  the 
forefather  to  his  descendants  of  dormant  gemmules,  which 
occasionally  become  developed  under  certain  known  or  unknown 
conditions.  Each  animal  and  plant  may  be  compared  with  a  bed 

1  For  a  complete  exposition  of  the  theory  see  Darwin's  "Animals  and  Plants 
under  Domestication"  (Ed.  2,  Vol.  II.,  Chapter  XXVII.). 


PANGENESIS  1G5 

of  soil  full  of  seeds,  some  of  which  soon  germinate,  some  lie 
dormant  for  a  period,  whilst  others  perish."  1 

Darwin  did  not  recognize  the  modern  distinction  between 
somatogenic  characters,  which  are  acquired  by  the  body  or  soma 
during  its  individual  life-time,  and  blastogenic  or  germinal 
characters,  which  are  supposed  to  originate  nf  the  germ  cells ;  or 
rather,  in  accordance  with  his  theory  of  pangenesis,  he  believed 
that  somatogenic  modifications  might  be  transferred  to  the  germ 
cells  and  thus  become  blastogenic.  In  other  words,  he  was  a 
firm  believer  in  the  inheritance  of  acquired  characters,  a  doctrine 
which,  as  we  shall  see  presently,  is  now  much  discredited,  and 
he  endeavoured  to  explain  by  means  of  his  theory  how  such 
characters  may  be  transmitted  from  parent  to  offspring. 

Suppose  some  part  of  the  body  in  a  particular  multicellular 
individual  were  to  become  modified  by  use  or  disuse,  or  by  the 
direct  action  of  the  environment.  Then  the  gemmules  given  off 
from  the  modified  cells  would  also  be  affected  in  a  corresponding 
manner  and  would  carry  information  of  the  change  to  the  germ 
cells.  It  would  be  as  if  some  constituency  with  many  repre- 
sentatives changed  its  political  opinions  and  instead  of  sending 
conservative  members  to  the  House  of  Commons  took  to  sending 
liberals.  When  the  proper  time  came  the  new  representatives 
would  vote  according  to  their  instructions ;  but  we  must  also 
suppose  that  the  old  ones  could  never  be  turned  out  and  that 
there  would  be  a  struggle  between  the  two.  At  first  the  old  ones 
would  be  the  more  numerous  and  would  outvote  the  new  ones; 
presently,  however,  the  new  ones,  being  constantly  reinforced, 
would  come  to  outnumber  the  old  ones  and  perhaps  be  able  to 
give  effect  to  the  altered  views  of  their  constituency.  As  Darwin 
says,  "It  is  generally  necessary  that  an  organism  should  be 
exposed  during  several  generations  to  changed  conditions  or 
habits,  in  order  that  any  modification  thus  acquired  should 
appear  in  the  offspring."  It  would  probably  be  more  in  accord 
with  the  facts  if,  instead  of  "  several  generations "  we  said 
"  a  large  number  of  generations."  In  this  sense,  we  may 
'well  believe  that  acquired  characters  can  be  inherited,  without 
expecting  to  be  able  to  demonstrate  such  inheritance  by  cutting 
off  the  tails  of  a  few  generations  of  mice. 

The  theory  of  pangenesis  certainly  explains  a  great  deal,  but 
it  involves  so  many  unprovable  assumptions  as  to  the  nature 

1  Loc.  cit.,  pp.  398-9. 


166        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

and  behaviour  of  the  "  gemmules  "  that  it  cannot  be  accepted  as 
more  than  what  Darwin  himself  termed  it,  a  provisional 
hypothesis  or  speculation. 

It  is  interesting  to  observe  that  Darwin  finds  the  cause  of 
variation  in  the  direct  influence  of  the  environment,  including 
under  that  term  the  effects  of  use  and  disuse  upon  the  organism. 
In  this  respect  he  agrees  with  the  views  of  Lamarck  and  differs 
widely  from  those  of  Weismann  and  many  other  modern 
biologists,  who  deny,  either  totally  or  in  part,  the  possibility  of 
the  inheritance  of  acquired  or  somatogenic  characters.  It 
will  have  been  noticed  that  the  theory  of  pangenesis  is  of  an 
essentially  pre-formationist  characterTfoTrE  assumes  the  existence, 
within  the  fertilized  egg,  of  an  immense  number  of  material 
particles  (gemmules)  which  in  some  way  or  other  represent  the 
different  inheritable  characters  of  the  body. 

The  celebrated  theory  of  heredity  which  we  owe  to  Professor 
Weismann l  is  based  upon  what  he  terms  the  "  Continuity  of 
the  Germ  Plasm."  The  general  idea  of  continuity  is,  of  course, 
by  no  means  a  new  one  ;  indeed  the  protoplasmic  continuity  of 
parent  and  offspring,  through  the  germ  cells,  must  form  the 
material  basis  for  the  transmission  of  characters  on  any  theory 
of  heredity.  Weismann,  however,  gives  much  greater  precision 
to  the  idea  than  any  of  his  predecessors.  He  identifies  the 
chromatin  of  the  nucleus  as  the  actual  hereditary  substance,  the 
bearer  of  all  inherited  tendencies,  and  draws  a  very  sharp  dis- 
tinction between  the  somatic  cells  which,  with  almost  endless 
diversity  of  form  and  function,  build  up  the  body  of  one  of  the 
higher  plants  or  animals,  and  the  germ  cells,  which  play  little  if 
any  part  in  the  life  of  the  individual  in  which  they  are  lodged 
but  are  destined,  under  favourable  circumstances,  to  give  rise  to 
the  next  generation. 

We  have  already  seen  that  the  germ  cells  are  frequently 
separated  from  the  somatic  cells  at  a  very  early  stage  in  develop- 
ment. It  may  be  that  the  distinction  between  the  two  is  actually 
inaugurated  by  the  very  first  division  of  the  fertilized  ovum,  as 
in  the  horse  worm,  Ascaris  megalocephala  (p.  129),  or  it  may  be 
recognizable  at  the  gastrula  stage,  as  in  the  arrow  worm,  Sagitta 

1  For  full  accounts  of  this  theory  see  the  English  translations  of  Weismann's  two 
chief  works  on  the  subject,  "  The  Germ  Plasm "  (Contemporary  Science  Series, 
1893)  and  "The  Evolution  Theory"  (Edward  Arnold,  1904). 


WEISMANN'S   THEOBY   OF   HEREDITY  167 

(p.  130),  but  such  cases  appear  to  be  very  exceptional  and  the 
segregation  of  the  germ  cells  usually  takes  place  much  later  and  at 
different  stages  of  the  development  in  different  species  of  plants 
and  animals.  The  exact  time  at  which  the  separation  of  the  two 
groups  of  cells  takes  place,  however,  does  not  seriously  affect  the 
argument.  In  any  case  the  ultimate  distinction  between  germ 
cells  and  somatic  cells  is  supposed  to  lie  in  the  fact  that  the 
former  retain  each  a  complete  sample  of  the  ancestral  germ 
plasm,  in  which  at  any  rate  all  the  essential  characters  of  the 
organism  are  in  some  way  or  other  represented,  while  the  latter, 
by  a  series  of  differential  divisions,  gradually  undergo  a  further 
segregation  into  the  different  tissue  cells  of  the  adult,  each  of 
which  contains  (in  an  active  condition)  only  a  sample  of  that 
part  of  the  ancestral  germ  plasm  which  is  appropriate  to  its  own 
particular  requirements. 

Hence  the  germ  cells,  complete  in  themselves  like  so  many 
Protista,  alone  retain  the  power  of  giving  rise  to  new  generations 
of  complete  individuals.  The  somatic  cells  have  sacrificed  this 
power  to  their  need  for  specialization.  Certain  phenomena, 
however,  such  as  the  regeneration  of  lost  parts  in  many  animals 
and  the  propagation  of  plants  by  buds  and  cuttings,  necessitate 
the  supposition  that  some  at  any  rate  of  the  somatic  cells  must 
retain  a  more  complete  sample  of  the  ancestral  germ  plasm  than 
is  necessary  for  their  own  development.  For  the  hereditary  sub- 
stance of  any  particular  cell  Weismann  adopts  Nageli's  term 
"  idioplasm,"  and  in  order  to  account  for  the  phenomena  just 
referred  to  he  is  obliged  to  postulate  the  existence,  in  the 
somatic  cells  in  question,  of  "  accessory  idioplasm,"  which  is  only 
called  into  activity  under  exceptional  conditions,  as,  for  example, 
when  it  becomes  necessary  for  a  crab  to  regenerate  a  lost  limb 
or  for  an  earthworm  to  renew  a  portion  of  its  body  which  has 
been  bitten  off  by  a  bird  or  chopped  off  by  a  spade. 

Weismann's  theory  involves  the  assumption  of  great  complexity 
of  structure  in  the  germ  plasm,  which,  as  we  have  already 
seen,  he  identifies  with  the  chromatin  substance  of  the  nucleus 
of  the  germ  cells.  He  finds  it  necessary  to  assume  the  existence, 
not  only  of  "  determinants,"  which  correspond  more  or  less  closely 
to  Darwin's  gemmules,  and  each  of  which  is  supposed  to  be 
responsible  for  the  development  of  some  special  inherited  feature 
of  the  organism,  but  also  of  structural  units  respectively  of  a 
lower  and  a  higher  order.  Thus  each  determinant  is  supposed 


168       OUTLINES  OF   EVOLUTIONARY  BIOLOGY 

to  be  made  up  of  "  biophors,"  which  are  themselves  the  lowest 
vital  units,  but  each  of  which  is  in  turn  made  up  of  molecules  in 
the  chemical  sense  of  the  term ;  while,  on  the  other  hand,  the 
determinants  are  supposed  to  be  grouped  in  "  ids,"  each  of  which 
is  a  complete  ancestral  germ  plasm,  theoretically  sufficient  in 
itself  to  determine  all  the  different  characters  of  an  entire 
individual.  The  ids  may  in  some  instances  correspond  to  the 
chromosomes,  but  these  appear  generally  to  be  composite  bodies 
("  idants  ")  made  up  each  of  a  large  number  of  ids  (chromomeres). 
The  determinants,  and  of  course  the  biophors  also,  are  far  below 
the  limits  of  visibility  even  with  the  aid  of  the  most  powerful 
microscope;  the  ids,  however,  frequently  appear  during  the 
process  of  mitosis  and  may  give  the  spireme  thread  or  the  chro- 
mosomes into  which  it  divides  a  characteristic  beaded  appearance 
(Fig.  32,  B).  Biophors,  determinants,  ids  and  idants  must  all  be 
looked  upon  as  living  entities,  growing  by  the  absorption  of 
nutriment  and  multiplying  by  division. 

In  accordance  with  Weismann's  theory  the  germ  cells  them- 
selves may  be  regarded  as  so  many  unicellular  organisms,  which 
multiply  by  fission  and  periodically,  if  they  chance  to  meet  with 
mates,  conjugate  with  one  another.  Their  cytoplasm  as  well  as 
their  chromatin  is  directly  continuous  from  generation  to  genera- 
tion just  as  it  is  in  a  dividing  Amoeba,  and  theoretically  there  is 
no  reason  why  the  constant  succession  of  germ  cells  should  ever 
be  interrupted  by  death.  The  soma,  or  body,  however,  stands  in 
a  very  different  position.  It  may  be  regarded  as  a  kind  of 
appendage  thrown  off  from  the  chain  of  germ  cells  after  each 
conjugation,  and  resulting  from  the  fact  that  most  of  the  cells 
arising  from  the  segmentation  of  the  zygote  not  only  remain 
together  in  intimate  association  with  one  another  but  become 
specialized  in  various  directions  and  co-operate  with  one  another 
to  form  a  complex  multicellular  individual.  Having  exhausted 
its  powers  of  growth  and  renewal  this  individual  body  sooner  or 
later  dies  ;  but  the  germ  cells  periodically  renew  their  powers  of 
cell-division  by  conjugation  and,  under  favourable  conditions,  go 
on  for  ever. 

According  to  Weismann,  inherited  characters  are  transmitted 
not  from  soma  to  soma  but  from  germ  cell  to  germ  cell,  by  virtue 
ef  the  continuity  of  the  germ  plasm.  The  soma  has  little  if  any 
influence  upon  the  germ  cells  which  it  contains  beyond  that  which 
is  involved  in  supplying  them  with  protection  and  nourishment. 


WEISMANN'S   THEORY   OF   HEREDITY  169 

There  is  no  other  means  of  communication  between  the  soma 
and  the  germ  cells,  and  hence  somatogenic  characters,  which 
are  acquired  in  the  life-time  of  the  individual  body  as  the  direct 
result  of  the  action  of  the  environment  (including  use  and  disuse 
of  organs),  cannot  be  transmitted  to  the  germ  cells  and  therefore 
cannot  be  inherited.  The  only  characters  which  can  be  inherited 
are  blastogenic  characters,  which  arise  by  modification  of  the 
germ  plasm  in  the  germ  cells  themselves. 

This  denial  of  the  transmission  of  so-called  acquired  charac- 
ters constitutes  the  most  important  difference  between  the 
theories  of  heredity  propounded  by  Weismann  and  Darwin. 
Both  these  theories  postulate  the  existence  of  ultra-microscopical 
material  particles,  determinants  or  gemmules,  but  Weismann's 
theory  allows  of  no  transference  of  such  particles  from  soma  to 
germ  cell,  only  from  germ  cell  to  germ  cell  and  from  germ  cell  to 
soma.  There  is  supposed  to  be  no  mechanism  for  the  trans- 
mission of  somatogenic  characters  to  the  next  generation.  Accord- 
ing to  the  older  view  the  germ  cells  give  rise  to  the  soma  and  the 
soma  to  the  germ  cells  alternately.  According  to  the  newer  one 
the  germ  cells  give  rise  to  the  soma  and  at  the  same  time  to  the 
next  generation  of  germ  cells,  while  the  soma  gives  rise  to  nothing 
but  itself  and  ultimately  perishes. 

i  The  contrast  between  the  two  views  is  clearly  expressed  in  the 
accompanying  diagram  (Fig.  76),  in  which,  for  the  sake  of  sim- 
plicity, the  complication  introduced  by  the  process  of  conjugation 
of  the  germ  cells  has  been  ignored. 

The  inheritance  of  somatogenic  characters  being  denied,  Weis- 
mann is  obliged  to  seek  the  origin  of  variations  from  some 
source  other  than  the  action  of  the  environment  and  use  and- 
disuse.  We  shall  return  to  this  point  presently.  In  the  meantime 
we  must  point  out  that  Weismann's  theory  harmonizes  very  well 
with  the  phenomena  of  mitosis,  and  especially  with  the  remarkable 
modifications  of  those  phenomena  which  accompany  the  matura- 
tion of  the  germ  cells. 

The  entire  process  of  mitosis  serves  to  emphasize  the  import- 
ance of  the  chromatin  substance  of  the  nucleus.  It  is  evidently 
of  the  utmost  consequence  that  this  substance  should  be  accur- 
ately apportioned  between  the  daughter  cells.  We  accordingly 
find  the  elaborate  mechanism  of  centrosomes  and  nuclear  spindle, 
and  a  splitting  of  each  individual  chromosome,  which  takes  place 
longitudinally  when  the  chromosomes  themselves  happen  to  bo 


170        OUTLINES   OF   E VOLUTION AEY  BIOLOGY 

elongated  in  form.  If,  as  Weismann  maintains,  and  as  can 
actually  be  demonstrated  in  many  cases,  each  elongated  chromo- 
some is  made  up  of  a  row  of  chromomeres  or  ids,  which  may  be 
supposed  to  differ  to  some  extent  from  one  another  as  to  the 
determinants  which  they  contain,  it  is  obvious  that  longitudinal 
splitting  of  the  chromosome  is  the  only  way  in  which  a  qualita- 
tive as  opposed  to  a  mere  quantitative  division  of  the  hereditary 


YIG.  76. — Diagram  to  illustrate  the  contrast  between  Darwin's  Theory  of 
Pangenesis  and  Weismann's  Theory  of  the  Continuity  of  the  Germ 
Plasm. 

The  figures  represent  an  imaginary  organism  with  four  processes  given  off  from  the  soma 
or  body,  which  is  supposed  to  contain  only  a  single  germ  cell  (dotted).  Three  genera- 
tions are  represented,  and  for  the  sake  of  simplicity  the  complication  introduced 
by  the  periodical  conjugation  of  male  and  female  germ  cells  is  omitted.  Figs.  A, 
B,  C  show  how  an  acquired  character — the  elongation  of  one  of  the  processes  as  a 
result  of  its  use  for  some  special  purpose — may  be  supposed  to  affect  the  germ  cells 
through  the  migration  of  gemmules  (indicated  by  the  small  arrows),  and  thus  be 
transmitted  by  heredity  in  accordance  with  the  theory  of  Pangenesis.  Figs.  A',  B', 
C'  show  how  such  an  acquired  character  is,  in  accordance  with  Weismann's  theory, 
unable  to  make  any  impression  upon  the  germ  cells  and  is  therefore  not  transmitted 
by  heredity.  In  the  first  case  the  germ  cells  of  each  generation  are  supposed  to  arise 
from  the  soma;  in  the  second  case  they  are  supposed  to  arise  directly  from  the 
preceding  generation  of  germ  cells,  which  also  gives  rise  to  the  soma  in  which  they 
are  enclosed,  as  indicated  by  the  large  arrows. 

substance  can  be  brought  about.  This  mode  of  division,  then, 
otherwise  difficult  to  explain,  is  fully  intelligible  in  accordance 
with  Weismann's  theory. 

It  is  not  necessary  to  suppose,  however,  that  such  division 
always  results  in  identical  daughter  chromosomes.  We  may 
assume  either  that  all  the  individual  ids  divide  into  similar 
halves,  containing  similar  determinants  (as  represented  very 
diagrammatically  in  Fig.  77,  A),  in  which  case  the  two  daughter 
chromosomes  will  be  exactly  alike,  or  that  some  or  all  of  the  ids 


WEISMANN'S   THEORY  OF   HEREDITY 


171 


divide  each  into  two  dissimilar  halves  containing  different  deter- 
minants (Fig.  77,  B),  when  the  daughter  chromosomes  will  be 
unlike  each  other.  In  the  former  case  the  division  is  said  to  be 
integral  and  in  the  latter  differential.  It  is  by  differential  division 
that  Weismann  believes  the  histological  differentiation  of  the 
sorna  to  be  brought  about. 

When  we  consider  the  phenomena  of  maturation  and  fer- 
tilization we  find  them  in  no  less  striking  harmony  with 
Weisnaann's  views.  We  have  already  pointed  out,  in  Chapter  X., 
that  .each  particular  species  of  plant  or 
animal  is,  as  a  general  rule,  characterized 
by  the  appearance  of  a  definite  and  con- 
stant number  of  chromosomes  in  all  the 
cells  of  the  body  during  the  process  of 
mitosis.  At  some  period  in  the  life-cycle, 
however,  in  typical  plants  during  the 
process  of  spore-formation  and  in  animals 
during  the  maturation  of  the  ova  and 
spermatozoa,  this  number  is  reduced  to 
half  by  separation  of  the  entire  chromo- 
somes into  two  groups,  one  of  which 
passes  to  each  of  two  daughter  cells. 
Thus  the  mature  germ  cells  have  only 
half  the  number  of  chromosomes  charac- 
teristic of  the  species  (or,  in  the  case  of 
typical  plants,  of  the  sporophyte  genera- 
tion). The  full  number  is  made  up  again 
by  the  union  of  male  and  female  gametes 
to  form  the  zygote  or  fertilized  egg. 

To  the  combination  of  the  maternal  and  paternal  chromosomes 

(in  the  nucleus  of  the  zygote  Weismann  has   given  the  name 

ymphimixis,  and   he  sees  in  this  mingling  of  ancestral   germ 

'plasms  the   cause   of  that  mixture    of    paternal  and  maternal 

Jcharacters  which  we  commonly  find  in  animals  and  plants.     If,  for 

the  sake  of  simplicity,  we  imagine  that  each  chromosome  consists 

(as  appears  to  be  sometimes  the  case)  of  only  a  single  id  or 

chromornere,  and  that  eight  of  these  are  present  in  the  nucleus  of 

the  mature  germ  cell,  we  may  represent  the  effect  of  repeated 

amphimixis   upon   the   constitution  of  the   nucleus   by  means 

of  the  diagram  (Fig.  78),  which  shows  how  the  ids  must  become 

more  and  more  diversified  in  each  successive  generation.     In  this 


FIG.  77.  —  Diagram  of 
(A)  integral  and  (B) 
differential  Division 
of  a  Chromosome  con- 
sisting of  five  Ids  or 
Chromomeres. 


172        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

diagram  A  represents  an  unreduced  nucleus  composed  of  sixteen 
ids,  eight  paternal  and  eight  maternal,  all  the  paternal  and  all  the 
maternal  ids  respectively  being  supposed  to  be  alike.  After 
reduction  and  union  with  another  mature  germ  cell  containing 
also  only  one  kind  each  of  paternal  and  maternal  ids,  but  both 
differing  in  some  respect  from  those  of  its  mate,  the  nucleus  of 
the  next  generation  will  contain  four  kinds  of  ids,  two  paternal 


m 


*J 


FIG.  78. — Diagram  illustrating  the  Composition  of  the  Germ  Plasm 
(Chromatin  Substance  of  the  Nucleus)  out  of  Ancestral  Ids,  and  the 
Effect  thereon  of  repeated  Amphimixis.  (From  Weismann's  "  Evolution 
Theory.") 

A — D  the  unreduced  nucleus,  containing  sixteen  chromosomes  (each  consisting  of  a 
single  id),  of  four  successive  generations.  In  A  the  germ  plasm  consists  of  only  two 
kinds  of  ids ;  in  B  of  four ;  in  G  of  eight,  and  in  D  of  sixteen.  mJ,  pJ,  maternal 
and  paternal  ids. 

and  two  maternal,  as  shown  in  B,  while  in  the  next  generation 
there  may  be  eight  kinds,  as  shown  in  C,  and  in  the  next  sixteen, 
as  in  D. 

We  must,  then,  in  accordance  with  the  views  of  Weismann, 
look  upon  the  germ  plasm  of  any  one  of  the  highar  organisms  as 
being  made  up  of  a  larger  or  smaller  number  of  ids,  each  one 
representing  the  inheritance  received  from  some  more  or  less 
remote  ancestor  on  the  paternal  or  maternal  side,  though  of  course 


WEISMANN'S   THEORY  OF   HEREDITY  173 

it  is  quite  possible  that  there  may  be  a  number  of  ids  of  the  same 
kind.  When,  during  the  process  of  maturation,  half  the  total 
number  of  chromosomes  (each  made  up  of  one  or  more  ids)  are 
eliminated  from  the  germ  cell,  it  appears  to  be  largely  a  matter 
of  chance  which  shall  go  and  which  shall  remain,  and  the  nature 
of  the  new  combination  of  ids  resulting  from  the  process  of 
amphimixis  must  also  be  a  matter  of  chance,  depending  upon 
what  luck  the  germ  cells  happen  to  have  in  their  mating. 

As  has  been  well  said,  a  new  shuffling  of  the  cards  must  take 
place  in  each  generation.  The  characters  of  the  organism 
developed  from  any  zygote  will  depend  upon  the  hand  dealt  out 
to  it  in  the  processes  of  reduction  and  amphimixis,  and  as  it  can 
rarely,  if  ever,  happen  that  any  two  hands  will  be  exactly  alike,  so 
it  will  rarely,  if  ever,  happen  that  any  two  organisms,  however 
closely  related,  will  exactly  resemble  one  another  in  all  their 
characters.  Indeed,  the  only  cases  in  which  even  an  approxima- 
tion to  exact  resemblance  is  known,  at  any  rate  amongst  the 
higher  animals,  are  those  of  "identical"  twins,  and  the  explana- 
tion of  these  is  that  they  have  arisen  by  an  integral  division  of  a 
single  fertilized  ovum,  followed  by  separation  of  the  first  two 
daughter  cells  or  blastomeres  thus  produced. 

We  therefore  find  in  the  processes  of  reduction  and  amphimixis, 
in  the  permutation  and  combination  of  ancestral  characters,  an 
abundant  source  of  variation.  This,  however,  is  not  supposed 
to  explain  fully  the  origin  of  variations,  and  Weismann  accord- 
ingly invokes  the  aid  of  another  hypothesis,  his  theory  of 
"  Germinal  Selection."  In  accordance  with  this  theory  the 
determinants  of  which  the  ids  are  composed  are  supposed  to  be 
differently  situated  with  regard  to  their  facilities  for  obtaining  the 
nutriment  necessary  for  their  growth  and  multiplication.  There 
is  a  kind  of  struggle  for  existence  going  on  amongst  them.  Those 
which  are  more  successful  in  obtaining  supplies,  having  once  got 
a  start,  will  tend  to  supplant  those  which  are  less  successful. 
Some  will  become  weaker  and  some  stronger,  and  thus,  as  the 
result  of  differences  in  nutrition,  variation  is  set  up  amongst  the 
determinants  themselves.  If  these  vary  it  follows  that  their 
determinates,  or  the  parts  which  they  control  in  the  developing 
organism,  will  vary  also.1 

1  There  would  seem,  however,  to  be  a  serious  objection  to  the  theory  of  germinal 
selection  in  the  fact  that  the  nucleus  of  any  given  germ  cell  contains  many  ids,  and 
that  similar  determinants  must  as  a  rule  recur  in  each  id.  We  can  hardly  suppose 
that  the  corresponding  determinants  in  each  id  are  always  subject  to  precisely  the 


174        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

In  this  way  Weismann  seeks  to  avoid  the  necessity  of  believing 
in  the  transmission  from  parent  to  offspring  of  modifications 
which  result  from  the  direct  action  of  the  environment  upon  the 
body  itself,  without,  however,  altogether  denying  that  external 
influences,  and  especially  nutrition,  may  act  upon  the  germ  plasm 
through  the  body  and  thus  cause  modifications  in  the  offspring. 

We  have  seen  that  Weismann's  theory  of  the  continuity  of  the 
germ  plasm  involves  the  acceptance  of  the  chromatin  of  the 
nucleus  as  the  actual  material  basis  of  hereditary  transmission. 
There  can  be  no  doubt  that  the  phenomena  of  ordinary  mitosis 
in  the  case  of  tissue  cells,  and  those  of  maturation  and  fertiliza- 
tion in  the  case  of  the  germ  cells,  strongly  support  this  view, 
while  the  simple  fact  that  the  spermatozoon,  consisting  almost 
entirely  of  chromatin  substance  and  with  a  minimum  of  cyto- 
plasm, contributes  equally  with  the  ovum  to  the  characters  of  tne 
offspring  in  normal  cases,  seems  almost  conclusive  as  to  the  pre- 
dominating importance  of  the  chromatin  in  this  respect.  Some 
observers,  however,  still  maintain  that  the  cytoplasm  plays  a  very 
important  part  in  heredity. 

It  is  probable  that  much  light  will  be  thrown  upon  this 
question  by  the  development  of  that  extremely  important  branch 
of  biological  science  known  as  experimental  embryology,  which  is 
as  yet  in  its  infancy.  We  have  already  pointed  out  that  in  certain 
cases  eggs  containing  no  nucleus  may  be  fertilized  by  spermatozoa, 
and  may  then  develop  up  to  a  certain  point,  and  that  this  process 
is  termed  merogony.  Some  years  ago  Boveri  claimed  to  have 
fertilized  enucleate  fragments  of  the  eggs  of  one  genus  of  sea- 
urchins  (Sphaerechinus)  with  the  sperm  of  another  genus 
(Echinus),  and  obtained  larvae  with  only  paternal  cfraracters. 
He  concluded  from  this  experiment  that  the  nuclear  substance  is 
alone  responsible  for  the  transmission  of  inherited  characters. 
Unfortunately  it  seems  that  his  results  are  open  to  a  different 
interpretation,  and  they  have  been  severely  criticized.  They 
certainly  cannot  be  regarded  as  by  any  means  conclusive. 

More  recently  Godlewski  has  succeeded  in  fertilizing  eggs  of 
the  common  sea-urchin  (Echinus)  with  sperm  of  the  feather  star 
(Antedon),  belonging  not  only  to  a  distinct  genus  but  to  a  widely 

same  advantages  or  disadvantages  of  position.  It  seems  much  more  likely  that 
variations  in  this  respect  in  different  ids  would  tend  to  neutralize  one  another,  the 
kind  of  determinant  which  is  unfavourably  situated  in  one  id  being  favourably 
situated  in  another,  so  that  eaeb.  kind  would,  on  an  average,  have  the  same  chance 
of  nutrition. 


EXPERIMENTS   IN   HEREDITY  175 

different  order  of  echinoderms.  The  larvae  of  these  two  types 
are  easily  distinguished  even  in  early  stages  of  development.  The 
larvae  produced  by  fertilization  of  normal  eggs  of  Echinus  with 
sperm  of  Antedon  are  said  to  be  of  the  maternal  type.  This 
result  is  in  itself  very  remarkable,  but  Godlewski  was  ajso  able  to 
fertilize  enueleate  egg  fragments  of  the  sea-urchin  with  sperm  of 
the  feather  star.-  Eight  larvae  produced  in  this  way  reached  the 
blastula  stage,  but  only  four  developed  as  far  as  the  gastrula. 
These  four,  however,  were  again  of  the  maternal .  type,  and  C9uld 
only  be  distinguished  by  their  size  from  those  of  the  pure  Echinus 
culture.  From  these  experiments  Godlewski  concludes  that 
cytoplasm  as  well  as  chromatin  must  be  concerned  in  the  trans- 
mission of  hereditary  characters,  for  no  maternal  chromatin  was 
present  in  the  eggs  from  which  the  larvae  developed.  Whether 
these  results,  which  are  in  direct-  opposition  to  those  of  Boveri, 
will  be  confirmed  or  refuted  by  further  observations  remains  to 
be  seen.  Walker,  iri  his  recent  work  on  heredity,1  has  accepted 
Godlewski's  conclusions  and  made  use  of  them  in  support  of  the 
theory  that  the  chromosomes  are  the  bearers  of  individual 
characters  only,  while  racial  characters  may  be  transmitted  by 
"  the  whole  protoplasm  of  the  cell." 

1  "Hereditary  Characters  and  their  Modes  of  Transmission,"  by  C.  E.  Walker 
(London,  Edward  Arnold,  1910). 


CHAPTER  XIII 

The  inheritance  of  acquired  characters  and  the  mnemic  theory  of  heredity. 

No  biological  question  during  the  last  fifty  years  has  given  rise 
to  more  acute  and  vigorous  controversy  than  that  of  the  inherit- 
ance or  non-inheritance  of  "  acquired  "  characters.  The  following 
paragraph  may  be  quoted  to  show  the  manner  in  which  Weismann 
stated  the  case  and  endeavoured  to  give  precision  to  the 
terminology  employed : — 

"  By  acquired  characters  I  mean  those  which  are  not  preformed 
in  the  germ,  but  which  arise  only  through  special  influences 
affecting  the  body  or  individual  parts  of  it.  They  are  due  to  the 
reaction  of  these  parts  to  any  external  influences  apart  from  the 
necessary  conditions  for  development.  I  have  called  them  '  somato- 
genic '  characters,  because  they  are  produced  by  the  reaction  of 
fche  body  or  soma,  and  I  contrast  them  with  the  ( blastogenic' 
characters  of  an  individual,  or  those  which  originate  solely  in 
the  primary  constituents  of  the  germ  ('  Keimesanlagen  ').  It  is  an 
inevitable  consequence  of  the  theory  of  the  germ-plasm,  and  of 
its  present  elaboration  and  extension  so  as  to  include  the  doctrine 
of  determinants,  that  somatogenic  variations  are  not  transmissible, 
and  that  consequently  every  permanent  variation  proceeds  from 
the  germ,  in  which  it  must  be  represented  by  a  modification  of 
the  primary  constituents." 1 

As  already  pointed  out,  the  view  here  expressed  is  directly 
opposed  to  that  of  Lamarck  and  Darwin,  who  believed  that 
characters  acquired  in  the  life-time  of  the  individual,  either  as  a 
result  of  the  use  or  disuse  of  organs  or  of  the  direct  action  of  the 
environment,  might  be  handed  on  by  heredity  from  parent  to 
offspring,  and  Darwin's  theory  of  pangenesis  was  essentially  an 
endeavour  to  imagine  some  mechanism  by  which  such  trans- 
ference of  acquired  characters  might  be  effected. 

We  may  at  once  agree  with  Weismann  that  blastogenic 
characters  alone  are  transmitted  from  parent  to  offspring,  but  the 
real  question  is— Can  a  somatogenic  character  be  converted  into, 

1  "  The  Germ-Plasm :  A  Theory  of  Heredity,"  by  August  Weismann  (Con* 
temporary  Science  Series,  1893),  p.  392 


D  $HAKA€TERS      17? 

ic   one?      In  : 

modi;  >f  the  body  or  sonia,  arib. 

individual  and  itself  in  no  way  due  tu  In; 
germ  cells   in   such  a  way  that   the   offspring   developed   from 
them  will  exhibit  a  corresponding  modification  of  its  soma  ? 

It  is  useless  either  to  deny  or  to  assert  the  possibility  of.  the 
inheritance  of  such  characters  on  any  purely  a  priori  grounds. 
The  fact  that  no  satisfactory  mechanism  for  the  transference  of 
such  characters  from  parent  to  offspring  h  •  yet  been  demon- 
strated does  not  justify  us  in  denying  the  possibility  of  such 
transference.  Our  decision  must  depend  upon  an  unbiassed 
examination  of  the  evidence  which  can  be  >rought  forward  on 
each  side. 

It  is,  of  course,  obvious  that  inasmuch  as  any  organism  differs 
to  a  greater  or  less  extent  from  its  ancestors,  the  differences 
being  as  a  general  rule  greater  in  proportion  to  the  remoteness 
of  the  particular  ancestor  with  which  it  is  compared,  the 
differentiating  characters  must  have  been  acquired,  in  "the 
ordinary  sense  of  the  word  during  the  interval  which  separates 
the  two  generations  in  question.  For  example,  there  can  be  no' 
reasonable  doubt  that  birds  are  descended  from  ancestors  which 
were  reptilian  in  character  and  had  no  featl  ^eatherfl  bive. 

unquestionably  been  acquired  somehow  or  other  during  the  pro- 
gress of  the  bird's  evolution.     This,  however,  is  not  the  sort  of 
acquisition  the  inheritance  of  which  is  in  dispute.    Weisman 
his  followers  would  deny  altogether  that  feathers  original 
somatogenic  characters ;  they  would  say  tin      certain  apparently 
fortuitous  modifications  in  the  constitution  of  the  germ  c 
themselves  were  responsible  for  the  first  app  arance  of  feat! 
probably  in  an  extremely  rudimentary  form — and  that  this,  ne^ 
character  proving  to  be  of  value  in  the  stru^ojt  i  <r  existence 
preserved  and  fostered  by  natural   selection  uu 
process  of  Devolution  the  elaborate  plumage  oi     as ting  b 
perfected. 

In   striking  qontrast  to  such  a  case  as  the  above  we 
innumerable  cases  of  the  more  or  less   sudden  appearance 
somatic  characters  during  the  life-time  of  an  individual  *•., 
obvious  result  of  the  action  of  some  external  or  environn 
influence,  or  of  the  use  or  disuse  of  some  organ  by  its 
and  it  is  to  such  cases  that  Weismann  and 
rightly  or  wrongly,  confine  the  discussion. 

B.  N 


178       OUTLINES   OF  EVOLUTIONARY   BIOLOGY 

Artificially  or  accidentally  produced  mutilations  afford  a  very 
good  example  of  such  obviously  somatogenic  characters,  and  with 
regard  to  the  inheritance  of  these  Weismann's  position  is  clearly 
summed  up  in  the  following  passage : — "  As  far  back  as  the 
eighteenth  century  the  great  philosopher  Kant,  and  in  our  own 
day  the  anatomist  Wilhelm  His,  gave  their  verdict  decidedly 
against  such  allegations,  and  absolutely  denied  any  inheritance 
of  mutilations ;  and  now,  after  a  decade  or  more  of  lively  debate 
over  the  pros  and  cons,  combined  with  detailed  anatomical 
investigations,  careful  testing  of  individual  cases,  and  experi- 
ment, we  are  in  a  position  to  give  a  decided  negative  and  say 
1  there  is  no  inheritance  of  mutilations." l  It  is,  however,  as 
\already  pointed  out,  all  a  question  of  evidence,  and  we  may  here 
quote  a  definite  case  in  order  to  show  the  nature  of  the  evidence 
with  which  we  have  to  deal : — 

^A  female  (and  very  prolific)  cat,  when  about  half-grown,  met 
with  an  accident.     '  Her  fine,  long  tail  was  trodden  on  and  had 
a  compound  fracture,  two  vertebrae  being  so  displaced  that  they  [ 
ever  after  formed  a  short  offset  between  the  near  and  far  end  of 
the  tail,  leaving  the  two  out  of  '  At  first  I  noticed  that  out 

of  every,  litter  of  kittens  some  had  a  tail  w,ith  a  querl  in  it.'    With 
'ssive  litters  ''-e  deformity  increased,  until    .iut  a  kitten  of 

had  a  straight  tail,  and  it  grew  worse  in  her  progeny  - 

i  now  wtfhave  not  a  cat  with  a  normal  tail  on  the  premises  ' 

n  a  cat-population  of  six  or  eight,  exclusive  of  young  kittens). 

tils  are  now  in  part  mere  stumps,  some  have  a  semicircular 

leways,  and  some  have  the  original  querl.     Perhaps  the 

'  was  somewhat  aggravated  by  in-and-in  breeding  and 

y  artificial  selection  practised  by  my  Chinaman,  who,  with  the 

perversity  of  his  race,  preferred   the   crooked   tails,  and   thus 

preserved  them  in  preference  to  the  normal  kittens.     There  arr 

no  other  abnormally-tailed  cats  in  the  neighbourhood.' " 

Professor  Brewer  quotes  this  remarkable  case2  from  "that 
keen  observer  and  eminent  scientist,  Professor  Eugene  W. 
Hii'gard  of  the  University  of  California,"  along  with  others  of  a 
like  kind  from  various  sources. 

It  is  of  course  essential  to  the  stability  of  Weismann's  theory 
•as  a  whole  that  evidence  such  as  this  should  be  rebutted.     I  am 
mot  aware  that  he  has  anywhere  criticized  this  particular  case, 
but  his  general  remarks  in   somewhat  similar  cases  may 
with  advantage  in  this  connection : — 

"The  Evolution  Theory"  (London,  1904),  Vol.  II.,  p.  65. 

Vide  Cope's  "  Primary  Factors  of  Organic  Evolution  "  (Chicago,  189G),  p  o.  4 32-3. 


INHERITANCE   OF   ACQUIRED  CHARACTERS     179 

"  In  the  first  place,  the  assertion  that  congenital  stump -tails 
in  dogs  and  cats  depended  on  inherited  mutilation  proved  to  be 
unfounded.  In  none  of  the  cases  of  stump-tails  brought  forward 
could  it  even  be  proved  that  the  tail  of  the  relevant  parent  had 
been  torn  or  cut  off,  much  less  that  the  occurrence,  in  parents  or 
grandparents,  of  short  tails  from  internal  causes  was  excluded. 
At  the  same  time  anatomical  investigation  of  such  stump-tails  as 
occur  in  cats  in  the  Isle  of  Man,  and  in  many  Japanese  cats,  and 
are  frequently  found  in  the  most  diverse  breeds  of  dogs,  showed 
that  these  had,  in  their  structure,  nothing  in  common  with  the 
remains  of  a  tail  that  had  been  cut  off,  but  were  spontaneous 
degenerations  of  the  whole  tail,  and  are  thus  deformed  tails,  not 
shortened  ones  (Bonnet). 

"  Experiments  on  mice_also  showed  that  the  cutting  off  of  the 
tail,  even  when  performed  on  both  parents,  does  ^ot  bring  about 
the  slightest  diminution  in  the  length  of  tail  in  the  descendants. 
I  have  myself  instituted  experiments  of  this  kind,  and  carried 
them  out  through  twenty-two  successive  generations,  without  any 
positive  result.  Corroborative  results  of  these  experiments  on 
nice  have  been  communicated  by  Ritzema  Bos  and,  indeperir- 
dently,  by  Rosenthal,  and  a  corresponding  series  of  experiments, 
on  rats,  which  these  two  investigators  carried  out,  yielded  the 
same  negative  results. 

"  When  wo  remember  that  all  the  cases  which  have  been 
brought  forward  in  support  of  an  inheritance  of  mutilations 
refer  to  a  single  injury  to  one  parent,  while,  in  the  experiments, 
the  same  mutilation  was  inflicted  on  both  parents  through 
numerous  generations,  we  must  regard  these  experiments  as  a 
proof  that  all  earlier  statements  were  based  either  on  a  fallacy 
or  on  fortuitous  coincidence.  This  conclusion  is  confirmed  by 
all  that  we  know  otherwise  of  the  effects  of  oft-repeated  mutila- 
^ons,  as  for  instance  the  well-known  mutilations  and  distortions 

'.ch  many  peoples  have  practised  for  long,  sometimes  incon- 
ably  long,  ages  on  their  children,  especially  circumcision,  the 

jaking  of  the  incisors,  the  boring  of  holes  in  lip,  ear,  or  nose, 
id  so  forth.  No  child  of  any  of  these  races  has  eyer  been 
/ought  into  the  world  with  one  of  these  marks ;  they  have  to  be 
re-impressed  on  every  generation."  ! 

It  cannot  be  seriously  questioned  that  in  the  majority  of  cases 
mutilations  are  not  visibly  inherited,  but  the  fact  that  no  one  has 
as  yet  succeeded  in  producing  experimentally  an  inheritable 
mutilation  does  not  prove  that  such  never  occur  accidentally. 

Weismann's  arguments  can  hardly  be  regarded  as  conclusive 
against  such  strong  evidence  as  that  afforded  by  Professor  Hilgard's 

•  Weismann,  "The  Evolution  Theory"  (London,  1904),  Vol.  II.,  pp.  65-6. 

N   2 


180        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

cats.  Moreover,  there  appears  to  be  considerable  difference  of 
opinion  as  to  whether  or  not  the  effects  of  circumcision  are  ever 
inherited.  Darwin,  in  discussing  this  point,1  admits  that  it  is 
possible  that  all  the  recorded  cases  of  apparent  inheritance  of 
such  effects  may  be  accidental  coincidences,  but  the  fact  that 
there  are  such  recorded  cases  prevents  us  from  accepting  the 
sweeping  generalization  which  Weismann  makes  in  the  last 
sentence  of  the  paragraph  above  quoted. 

Darwin  himself  was  convinced  that  the  effects  of  operations 
are  sometimes  inherited  by  the  well  known  and  often  quoted  case 
of  Dr.  Brown-Sequard's  guinea-pigs.  In  these  animals  not  only 
did  epilepsy  appear  in  the  offspring  of  parents  which  had  been 
rendered  epileptic  by  injury  to  the  nervous  system,  but  certain 
structural  modifications  (e.g.  loss  of  toes)  appeared  in  the 
offspring  when  corresponding  structural  modifications  had  been 
produced  in  the  parent  (indirectly)  by  similar  injury.  Weismann, 
however,  endeavours  very  ingeniously  to  explain  away  these 
observations,  and  the  correctness  of  Brown-Sequard's  results  has 
been  seriously  impugned. 

It  cannot  be  denied,  however,  that  the  primd facie  evidence  for 
the  occasional  transmission  of  such  characters  as  those  produced 
by  mutilation  is  very  strong.  We  may  allow  much  for  coinci- 
dence, but  well  authenticated  cases  like  that  of  the  cats  above 
referred  to,  and  like  the  following,  are  too  numerous  to  be  all 
explained  away  in  this  manner  : — 

"  A  person,  when  a  boy  of  ten  years,  cut  the  terminal 
phalange  of  the  little  finger  of  his  left  hand  with  a  sickle.  The 
joint  was  not  injured,  nor  was  the  function  of  the  finger  seriously 
impaired.  There  was,  however,  an  obvious  deformity.  The 
finger  was  ill-shaped  and  crooked,  and  the  nail  abnormal.  He 
married  and  had  two  children,  the  first  a  son,  with  normal 
fingers,  the  second  a  daughter,  who  had  the  little  finger  of  the 
corresponding  (the  left)  hand  deformed  from  birth  in  the  same 
manner.  The  function  of  the  finger  was  not  seriously  injured, 
but  the  deformity  was  precisely  the  same  in  shape,  even  to  the 
malformation  of  the  finger-nail.  She  died  at  thirty,  without 
children,  consequently  no  observation  on  a  succeeding  generation 
could  be  noted.  None  of  his  other  kindred  had  malformed 
fingers,  nor  had  any  ancestor  of  the  child  for  at  least  three 
generations,  and  there  was  no  knowledge  of  any  such  in  the 
more  remote  ancestry. 

»  "  Animals  and  Plants  under  Domestication  "  (2nd  Ed.,  1882),  Vol.  I.,  p.  467. 


INHERITANCE   OF  ACQUIRED   CHARACTERS     181 

"  (This  case  was  related  to  me  in  full  detail  by  the  father  with 
the  deformed  finger,  and  with  whom  I  was  personally  acquainted. 
He  was  an  eminent  physician,  the  president  of  a  large  and 
reputable  medical  college,  and  his  name  is  well  known  to  the 
profession.)  "  1 

It  seems  only  reasonable,  then,  to  admit  that  suddenly 
acquired  somatogenic  characters  are  occasionally,  though  probably 
very  rarely,  transmitted  by  heredity  in  a  sufficiently  pronounced 
form  to  be  recognizable,  but  even  if  this  were  not  so  we  should 
not  be  justified  in  concluding  that  all  the  characters  which  an 
organism  exhibits  have  their  origin  in  the  first  instance  in  modi- 
fications of  the  germ  plasm.  It  must  be  remembered  that  a  great 
many  somatogenic  modifications  are  the  result  of  what  we  may 
call  the  normal  action  of  the  environment,  and  that  such  action 
may  extend  over  very  long  periods  of  time,  in  which  many 
thousands  of  generations  may  be  produced.  In  such  cases  the 
stimulus,  whatever  it  may  be,  to  which  the  organism  responds  by 
modification  in  bodily  structure,  is  repeated  a  vast  number  of 
times,  and  therefore  seems  much  more  likely  to  produce  an 
inheritable  effect  than  a  single  accidental  or  experimental  injury. 
A  single  drop  of  water  falling  on  a  stone  makes  no  visible 
impression,  but  if  the  dropping  goes  on  for  a  long  time  the  stone 
will  gradually  be  worn  away,  and  it  is  by  no  means  essential  to  a 
belief  in  the  heritability  of  acquired  characters  that  such 
characters  should  be  immediately  inherited  in  full  perfection. 
It  may  be  merely  a  question  of  time. 

It  is  a  well  known  fact  that  the  "habit"  and  even  the 
structure  of  a  plant  are  largely  determined  by  the  conditions 
under  which  its  growth  takes  place.2  A  plant  growing  in  a  hot- 
house may  acquire  a  very  different  habit  from  another  of  the 
same  species  growing  in  the  open  air.  Many  alpine  or  sub-alpine 
plants  have  become  adapted  to  their  peculiar  environment  by 
various  highly  characteristic  modifications,  amongst  which  a 
reduction  in  the  size  of  the  leaves  is  one  of  the  most  conspicuous, 
and  such  plants  may  be  induced  to  change  their  mode  of  growth 
by  simply  removing  them  to  sufficiently  warm  and  sheltered 
situations,  in  which  their  habit  comes  to  approach  that  of  their 
lowland  relatives.  In  such  cases  it  is  difficult  to  say  how  far 

1  This  is  another  of  the  striking  cases  collected  by  Professor  Brewer  and  quoted 
in  Cope's  "  Primary  Factors  of  Organic  Evolution,"  pp.  433-4. 

2  The  reader  should  consult  the  quotations  from  Lamarck  bearing  upon  this  point 
given  in  Chapter  XXIV.,  especially  the  case  of  Ranunculus  aquatilis. 


182        OUTLINES   OF   EVOLUTIONABY  BIOLOGY 

the  habit  of  the  plant,  whether  alpine  or  lowland,  is  really 
inherited,  or  how  far  it  may  be  produced  afresh  in  each  generation 
as  a  direct  response  to  environmental  stimuli. 

It  appears,  however,  from  the  observations  of  Bordage,1  that 
peach  trees  in  the  climate  of  Keunion  gradually  acquire  an 
almost  evergreen  habit,  and  that  this  character  is  hereditarily 
transmitted,  being  exhibited  by  seedlings  of  the  modified  trees 
when  grown  in  situations  where  the  peach  is  usually  deciduous. 
Dr.  Bordage  considers,  and  it  appears  to  us  that  he  has  good 
reason  for  so  doing,  that  his  observations  and  experiments  defi- 
nitely prove,  in  the  case  of  plants,  the  hereditary  transmission 
of  characters  acquired  under  the  influence  of  a  change  of  climate. 

To  take  another  example,  it  has  been  shown  that  in  rats  and 
mice  certain  modifications  in  bodily  structure  are  produced  as  the 
result  of  raising  or  lowering  the  temperature  to  which  the  young 
animal  is  exposed  during  its  growth,  and  such  modifications  appear 
to  be  inherited.  Sumner 2  found,  as  the  result  of  a  large  number 
of  careful  measurements,  that  mice  reared  in  a  warm  room  (about 
21°  C.)  differed  considerably  from  those  reared  in  a  cold  room 
(about  5°  C.)  as  regards  the  mean  length  of  the  tail,  foot  and  ear, 
which  were  longer  in  the  former  than  in  the  latter.  The  same 
differences  occurred  to  a  recognizable  extent  in  the  offspring  of 
the  warm  room  and  cold  room  parents,  although  these  offspring 
were  all  reared  together  in  a  common  room  under  identical 
temperature  conditions. 

Observations  such  as  these,  which  are  rapidly  accumulating, 
lead  us  to  hope  that  the  question  of  the  inheritance  or  non- 
,  inheritance  of  somatogenic  characters  which  have  undoubtedly 
arisen  in  response  to  the  direct  action  of  the  environment 
may,  before  long,  be  answered  conclusively.  They  doubtless  require 
confirmation  and  extension,  but  they  afford  very  strong  evidence 
in  favour  of  the  inheritance  of  such  characters.  It  has  been  sug- 
gested that  in  such  cases  the  stimulus  of  changed  conditions  affects 
the  body  and  the  germ  cells  simultaneously  and  in  a  parallel 
manner,  rather  than  that  the  body  is  modified  first  and  then  in- 
fluences the  germ  cells,  but  such  a  suggestion  seems  like  a  last 
attempt  to  avoid  at  all  costs  the  necessity  for  believing  in  the  j 

1  Edmond  Bordage,  "A  propos  de  1'he'redite  des  caracteres  acquis  "  (Bulletin 
Scientifique  dela  France  et  de  la  Belgique.     Tome  XLIV,  Paris,  1910). 

2  Francis  B.  Sumner,  "An  Experimental  Study  of  Somatic  Modifications  and 
their  Reappearance  in  the   Offspring,"  (Archiv  fiir   Entwicklungsmechanik  der 

•  Organismen,  Bd.  80,  1910). 


INHEBITANCE  OF  ACQUIRED  CHARACTERS     183 

transmission  of  "  acquired  "  characters,  and  is  quite  as  difficult 
to  accept  as  the  latter,  while,  as  Sumner  points  out,  it  does  not 
affect  the  question  of  the  importance  of  the  environment  in 
determining  the  course  of  evolution. 

Professor  Henslow *  believes  that  the  direct  action  of  the 
environment,  in  ita  widest  sense,  coupled  with  the  responsive 
power  of  protoplasm,  is  the  sole  and  efficient  cause  of  adaptive 
variations  in  plants,  without  any  aid  from  natural  selection. 
This  of  course  implies  a  firm  belief  in  the  inheritance  of 
somatogenic  modifications.  Although  these  may  be  acquired 
slowly  throughout  the  course  of  a  long  series  of  successive 
generations,  they  may  become  gradually  more  and  more  fixed 
and  permanent,  until  finally  they  attain  a  degree  of  stability 
which  entitles  them  to  be  considered  as  truly  blastogenic. 

Professor  Eigenmann2  has  arrived  at  very  similar  conclusions 
as  a  result  of  his  careful  study  of  animals  which  live  in  dark 
caves.  A  very  characteristic  feature  of  such  animals  is  the 
bleaching  which  they  undergo,  due  to  the  loss  of  pigment.  Pro- 
fessor Eigenmann  regards  this  character  as  due  in  the  first  place 
to  the  direct  influence  of  the  environment  (i.e.  the  absence  of 
light)  upon  the  individual.  He  tells  us  "  The  bleached  condition 
of  animals  living  in  the  dark,  an  individual  environmental  adap- 
tation, is  transmissible,  and  finally  becomes  hereditarily  fixed." 
In  Amblyopsis,  one  of  the  blind  cave  fishes,  the  fixation  of  this 
character  has  become  so  complete  that  even  when  the  young  are 
reared  in  the  light  they  develop  without  pigment — the  originally 
somatogenic  character  has  apparently  become  blastogenic.  In 
the  well  known  European  Proteus — a  tailed  amphibian  which 
lives  in  subterranean  waters — on  the  other  hand,  it  appears  that 
the  bleached  condition  has  not  yet  become  hereditarily  established, 
for  this  animal  becomes  darker  when  exposed  to  the  light.  Pro- 
fessor Eigenmann  points  out  that  "  natural  selection  cannot 
have  affected  the  coloration  of  the  cave  forms,  for  it  can  be  of  no 
consequence  whether  a  cave  species  is  white  or  black." 

We  do  not  know  how  many  generations  it  may  take  to  effect 
the  fixation  of  a  character  acquired  slowly  under  the  influence 
of  a  constantly  repeated  or  continuous  environmental  stimulus, 
but  the  fact  that  human  beings  have  not  yet  learned  to  speak 

1  "  Origin  of  Plant  Structures  "  (International  Scientific  Series,  Vol.  LXXVIL, 
1895). 

2  "  Cave  Vertebrates  of  America.    A  Study  in  Degenerative  Evolution"  (Wash- 
ington :  Carnegie  Institution,  1909). 


184        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

without  being  taught  shows  that  the  number  may,  in  some  cases 
at  any  rate,  be  very  large.  Speech,  however,  is  a  comparatively 
recent  acquisition  in  the  human  race  and  many  of  the  higher 
animals  perform  actions  without  being  taught  which  their  ancestors 
may  very  well  have  originally  learnt  to  perform  by  observation 
or  experience.  It  would  be  extremely  difficult  to  explain  the 
nest-building  habits  of  birds,  and  other  so-called  "instincts,'4 
except  as  having  been  originally  acquired  in  this  way  in  the 
life-time  of  the  individual. 

On  the  whole,  then,  the  available  evidence  seems  to  indicate  that 
suddenly  and  exceptionally  acquired  characters,  such  as  mutilations, 
are  occasionally  but  only  rarely  inherited  to  such  an  extent  as  to  be 
recognizable,  while,  on  the  other  hand,  characters  which  are  due  to 
the  continued  action  of  some  external  stimulus,  extending  perhaps 
over  many  generations,  in  the  long  run  become  so  firmly  impressed 
upon  the  organism  that  they  affect  the  germ  cells  as  well  as  the 
somatic  cells  and  thus  become  truly  blastogenic — 
•—^  We  must  look  upon  the  germ  cells  as  highly  conservative 
bodies,  possessed  of  great  inertia,  which  can  only  be  induced  very 
slowly  in  normal  circumstances,  or  by  some  sudden  revolution, 
of  the  nature  of  which  we  know  nothing,  in  a'bnormal  circum- 
stances, to  alter  their  constitution.  This  conservatism  is  no 
doubt  of  great  value  as  a  check  upon  the  inheritance  of  accidental 
and  unfavourable  modifications.  The  adaptation  of  the  individual 
to  its  particular  environment  is  to  a  large  extent  provided  for  by 
f  somatogenic  modifications  which  arise  in  its  own  life-time.  What 
suits  one  individual,  however,  may  not  suit  its  successors,  which 
may  have  to  live  under  more  or  less  widely  different  conditions, 
and  for  progressive  evolution  what  is  needed  is  an  average 
adaptation  of  the  race  to  an  average  environment.  Hence  the 
»germ  plasm  in  its  evolution  lags  behind  the  soma  and  is  only 
very  slowly  impressed  with  those  characters  which  experience 
has  shown  to  be  of  permanent  value  to  the  race.  It  is  a  kind  of 
second  chamber  which  acts  as  a  useful  check  upon  too  radical 
tendencies. 

That  the  germ  plasm  is  to  some  extent  capable  of  modification  by 
environmental  stimuli,  however,  is  clearly  shown  by  the  experiments 
of  Tower  referred  to  in  Chapter  XL,  but  it  is  extremely  difficult  to 
see  how  such  stimuli  as  he  mentions  could  affect  it  except  by 
acting  primarily  upon  the  soma  in  which  the  germ  cells  are 
enclosed.  Then  the  soma  might  be  modified  in  response  to  the 


INHERITANCE  OF  ACQUIRED  CHARACTERS      185 

stimulus  in  the  first  instance,  and  this  modification,  whatever  it& 
nature  (and  it  might  of  course  be  quite  unrecognizable  by  us), 
might  affect  the  germ  cells— in  which  case  the  germ  cells  could  not 
be  entirely  shut  off  from  the  influence  of  the  soma. 

The  chief  obstacle  in  the  way  of  our  belief  in  the  inheritance  of 
acquired  characters  lies,  as  we  have  already  seen,  in  the  difficulty 
of  imagining  any  mechanism  adequate  to  bring  about  the  con- 
version of  somatogenic  into  blastogenic  modifications.  -  If,  as 
Weismann  insists,  the  germ  cells  are  really  incapable  of  being 
influenced  by  the  body,  the  difficulty  does  indeed  seem  insur- 
mountable ;  but  there  seems  to  be  no  valid  reason  why  we  should 
follow  Weismann  in  this  respect.  Nor  does  it  seem  necessary 
that  we'should  adopt  any  theory  which  postulates  the  migration 
of  material  particles,  such  as  the  gemmules  of  Darwin's  pan- 
genesis,  in  order  to  get  over  the  difficulty. 

Herbert  Spencer  long  ago  indicated  the  direction  in  which  the 
solution  of  this  problem  must  be  sought.  "  It  is,"  he  says,  "an 
unquestionable  deduction  from  the  persistence  of  force,  that  in 
every  individual  organism  each  new  incident  force  must  work  its 
equivalent  of  change ;  and  that  where,  it  is  a  constant  or  recurrent 
force,  the  limit  of  the  change  it  works  ^must  be  an  adaptation  of 
structure  such  as  opposes  to  the  new  outer  force  an  equal  inner 
force.  The  only  thing  open  to  question  is,  whether  such  re-adjust- 
ment is  inheritable  ^  and  further  consideration  will,  I  think,  show, 
that  to  say  it  is  not  inheritable  is  indirectly  to  say  that  force  does 
not  persist.  If  all  parts  of  an  organism  have  their  functions 
co-ordinated  into  a  moving  equilibrium,  such  that  every  part 
perpetually  influences  all  other  parts,  and  cannot  be  changed 
without  initiating  changes  in  all  other  parts — if  the  limit  of 
change  is  the  establishment  of  a  complete  harmony  among  the 
movements,  molecular  and  other,  of  all  parts ;  then  among  other 
parts  that  are  modified,  molecularly  or  otherwise,  must  be  those 
which  cast  off  the  germs  of  new  organisms.  The  molecules  of 
their  produced  germs  must  tend  ever  to  conform  the  motions  of 
their  components,  and  therefore  the  arrangements  of  their  com- 
ponents, to  the  molecular  forces  of  the  organism  as  a  whole ;  and 
if  this  aggregate  of  molecular  forces  is  modified  in  its  distribution 
by  a  local  change  of  structure,  the  molecules  of  the  germs  must 
be  gradually  changed  in  the  motions  and  arrangements  of  their 
components,  until  they  are  re-adjusted  to*the  aggregate  of  mole- 
cular forces.  For  to  hold  that  the  moving  equilibrium  of  an 


186        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

organism  may  be  altered  without  altering  the  movements  going 
on  in  a  particular  part  of  it,  is  to  hold  that  these  movements  will 
not  be  affected  by  the  altered  distribution  of  forces ;  and  to  hold 
this  is  to  deny  the  persistence  of  force." 1 

In  other  words,  the  whole  complex  system  of  forces  which 
determines  the  constitution  of  the  germ  cells  must  be  in  a  state 
of  equilibrium  with  the  system  of  forces  which  determines  the 
constitution  of  the  body,  and  any  disturbance  in  the  latter  must 
be  met  by  a  corresponding  disturbance  in  the  former.  The  same 
idea  may  be  expressed  by  saying  that  modifications  of  the  soma 
act  as  stimuli,  which  make  more  or  less  permanent  impressions  or 
"engrams"  upon  the  germ  cells  in  much  the  same  way  that 
*  stimuli  of  various  kinds  received  through  the  sense  organs  make 
impressions  upon  certain  cells  in  the  brain."1  >  In  the  case  of  the 
brain  cells  the  original  impressions  may  become  dormant  and  be 
revived  later  on  as  memories,  which  are  a  kind  of  reproduction 
of  the  phenomena  to  which  the  impressions  were  originally  due. 
In  the  case  of  the  germ  cells  it  is  supposed  that  the  engrams  also 
become  dormant,  but  are  aroused  to  activity  again  in  the  course 
of  the  development  of  these  cells  into  new  organisms,  exerting 
their  influence  in  such  a  manner  as  to  bring  about  a  kind  of 
reflection,  in  the  body  of  the  offspring,  of  the  parental  characters. 
This  is  the  central  idea  of  the  "Mnemic"2  theory  of  heredity, 
associated  more  especially  with  the  names  of  Hering,  Samuel 
Butler3  and  Semon.4 

/  According  to  some  authorities  it  is  through  a  continuity  of 
protoplasm  from  cell  to  cell,  or  especially  through  the  nervous 
system  in  the  case  of  the  higher  animals,  that  stimuli  are  trans- 
mitted from  the  soma  to  the  germ  cells,  but  the  assumption  of 
any  definite  material  paths  for  such  transmission  seems  an 
altogether  superfluous  encumbrance  of  the  theory. 

It  has  long  been  known  that  it  is  possible  to  send  messages,  or  if 
we  like  to  call  them  so,  stimuli,  from  one  place  ifo  another  without 
any  material  means  of  communication.  The  very  existence  of 
life  on  the  earth  depends  upon  stimuli  received  in  the  form  of 

«4    l  "  Principles  of  Biology  "  (London,  1881),  Vol.  II.,  p.  387. 

2  From  the  Greek  ju.vniJ.ri,  memory. 

«  See  Butler's  "Life  and  Habit"  (2nd  Edition,  London,  1878)  and  "Un- 
conscious Memory  "  (New  Edition,  London,  1910),  the  latter  of  which  contains  a 
translation  of  Bering's  lecture  "  On  Memory  as  a  Universal  Function  of  Organized 
Matter." 

*  Semon,  "  Die  Mneme  als  erhaltendes  Prinzip  im  Wechsel  des  organischcn 
Geschehens,"  3rd  Ed.,  Leipzig,  1911. 


INHERITANCE  OF  ACQUIRED  CHARACTERS     187 

light  and  heat  from  the  sun  across  an  intervening  space  of  almost 
inconceivable  extent  which  is  supposed  to  contain  nothing  but 
the  hypothetical  and  intangible  ether.  To  account  for  such 
transmission  we  postulate  the  occurrence  of  vibrations  or  undula- 
tions in  this  ether,  and  the  truth  of  this  theory  may  now  be 
regarded  as  conclusively  demonstrated.  The  stimulation  of  the 
sensory  cells  of  the  retina  by  certain  of  these  vibrations  gives  rise, 
when  transmitted  along  the  optic  nerve  to  certain  cells  in  the  brain, 
to  the  sensation  which  we  recognize  as  light,  while  other 
vibrations  are  perceived  by  us  as  heat. 

The  rapid  development  of  wireless  telegraphy  in  recent  years 
has  familiarized  us  with  another  type  of  vibration  in  the  ether, 
by  taking  advantage  of  which  we  are  able  to  send  messages  over 
immense  distances  from  one  instrument  to  another,  without  the 
aid  of  wires  or  any  other  material  connections.  The  Rontgen 
rays,  again,  can  make  impressions  upon  sensitive  photographic 
plates  after  passing  through  solid  objects  which  are  quite  imper- 
vious to  the  ordinary  light  vibrations. 

In  view  of  these  facts  it  seems  absurd  to  deny  that  the  living 
cells  of  the  soma,  in  which  doubtless  complex  vibrations,  possibly 
comparable  to  those  which  are  responsible  for  the  phenomena 
of  light  and  electricity,  are  constantly  going  on,  may  conceivably 
influence  the  germ  cells  without  our  being  able  to  demonstrate 
the  existence  of  material  connections  by  which  the  necessary 
stimuli  might  be  transmitted. 

The  different  means  of  communication  between  one  cell  and 
another  in  the  living  body  may  be  compared  to  the  different 
methods  by  which  messages  are  transmitted  between  distant 
members  of  a  civilized  human  community.  The  circulation  of 
fluids  (such  as  the  blood)  distributes  throughout  the  body  definite 
substances  secreted  by  certain  cells,  which  may  act  as  stimu- 
lants upon  other  and  far  distant  cells.  We  may  compare  this 
to  the  transmission  of  messages  by  letter  post.  The  nervous 
system  of  the  higher  animals,  and  probably  also  the  delicate  threads 
of  protoplasm  which  so  frequently  connect  one  cell  with  another 
both  in  animals  and  plants,  provide  a  means  of  communication 
which  is  closely  comparable  to  ordinary  telegraphy  over  conducting 
wires.  Why  should  we  deny  the  possibility  that  a  third  means 
of  communication,  analogous  to  wireless  telegraphy,  may  also 
exist  ? 

Darwin's     hypothesis     of     pan  genesis     depends     upon     the 


188        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

existence  of  the  first  of  these  means  of  communication.  So  also 
does  the  modern  theory  of  hormones,  chemical  substances  which 
are  believed  to  exercise  a  most  important  controlling  influence 
over  parts  of  the  body  that  may  be  far  remote  from  the 
place  where  they  themselves  are  secreted,1  and  the  aid  of 
which  has  also  been  invoked  to  account  for  the  phenomena 
of  heredity.  Others,  again,  as  we  have  seen,  have  called  in  the 
aid  of  the  second  means  of  communication  to  account  for  the 
transmission  of  stimuli  from  somatic  to  germ  cells,  but  their 
view  is  hardly  supported  by  what  we  know  of  the  arrangement  of 
the  nervous  system  throughout  the  animal  kingdom,  and  the 
difficulty  of  accepting  it  is  still  greater  in  the  case  of  plants,  in 
which  no  definite  nervous  system  is  developed.  The  third  method 
of  communication,  however,  seems  open  to  no  objection,  and, 
whether  it  may  be  supplemented  by  the  others  or  not,  seems 
amply  sufficient  to  account  for  the  facts. 

The  faculty  of  receiving  and  responding  to  stimuli  of  various 
kinds  is  one  of  the  most  characteristic  features  of  living  proto- 
plasm. In  the  higher  animals  this  function  is  more  or  less 
concentrated  in  definite  sense  cells,  each  of  which  is  adapted  for  the 
reception  of  stimuli  of  one  particular  kind.  The  visual  sense  cells 
are  adapted  for  the  reception  of  stimuli  from  the  light-vibrations 
of  the  ether,  the  auditory  sense  cells  are  stimulated  by  vibrations  of 
the  air  or  water  in  which  the  animal  lives,  the  olfactory  sense  cells 
are  stimulated  by  the  chemical  action  of  material  particles,  and 
the  tactile  cells  by  the  mechanical  stimuli  of  contact  and  pressure. 
In  such  an  organism  as  the  unicellular  Amceba,  on  the  other 
hand,  all  parts  of  the  superficial  protoplasm  are  probably  sensitive 
to  stimuli  and  no  special  organs  of  sense  are  developed.  We  may 
well  suppose,  therefore,  that  the  undifferentiated  germ  cells  of  the 
multicellular  animals  and  plants  are  sensitive  to  stimuli  received 
from  the  somatic  cells,  though  it  is  impossible  in  the  present  .state 
of  our  knowledge  to  determine  the  nature  of  these. 

It  is  not  difficult  to  demonstrate  that  one  cell  may  actually  be 
stimulated  by  another  without  the  existence  of  any  protoplasmic 
connection  between  the  two,  as,  for  example,  in  the  mutual 
attraction  of  male  and  female  gametes.  Such  stimulation,  it  is 
true,  is  usually  attributed,  mainly  at  any  rate,  to  the  secretion 
of  some  specific  chemical  substance  which  diffuses  into  the  sur- 
rounding water,  and  classed  accordingly  under  the  head  of 

1  Vide  p.  126. 


, 
INHERITANCE  OF  ACQUIRED  CHARACTERS      189 

cbemotaxis.  In  the  case  of  Spirogyra,  however,  discussed  in 
Chapter  X,1  it  certainly  seems  as  if  chemotaxis  were  not  sufficient 
to  account  for  the  phenomena. 

Belief  in  the  possibility  of  the  transmission  of  acquired 
characters  from  somatic  to  germ  cells  by  no  means  obliges  us  to 
throw  over  Weismann's  theory  of  determinants.  Indeed  it  seems 
necessary  to  postulate  the  existence  in  the  germ  plasm  of  some 
such  material  priraordia  as  the  responsible  agents  in  the  trans- 
mission of  heritable  characters  in  order  to  account  for  the 
Mendelian  phenomena  of  hybridism,  and  especially  the  existence 
of  interchangeable  unit  characters,  which  will  be  dealt  with  in 
the  following  chapter.  Moreover,  the  theory  of  determinants,  as 
we  have  already  seen,  harmonizes  very  well  with  what  we  know 
of  the  microscopic  structure  of  the  germ  plasm  and  its  behaviour 
in  cell-division  and  conjugation. 

It  may  be  that  it  is  the  determinants  themselves  that  are 
influenced  by  stimuli  received  from  the  somatic  cells.  It  is  even 
possible  that  each  different  kind  of  determinant  is  "  tuned " 
to  respond  to  vibrations  of  a  particular  wave-length,  emanating 
from  corresponding  determinants  in  the  nuclei  of  somatic  cells  of 
a  particular  kind.  Modifications  of  these  somatic  cells  may  then, 
by  altering  the  character  of  the  vibrations  in  the  determinants  of 
their  own  nuclei,  affect  the  corresponding  determinants  in  the 
distant  germ  cells  in  a  similar  manner.  By  means  of  some  such 
hypothesis  as  this,  which  would  be  strictly  in  accordance  with 
modern  physical  ideas  on  the  subject  of  the  transmission  of 
electrical  energy  from  one  group  of  electrons  to  another  at  a  dis- 
tance, any  a  priori  difficulty  in  accepting  the  possibility  of  the 
inheritance  of  acquired  characters  might  be  removed. 

The  case  of  neuter  insects  has  frequently  been  adduced  as  a 
serious  stumbling  block  in  the  way  of  the  theory  of  the  origin  of 
permanent,  heritable  characters  from  somatogenic  modifications, 
for  these  neuters  possess  features  of  their  own  which  are  not 
shared  by  the  males  and  perfect  females  and  which,  as  the 
neuters  do  not  themselves  produce  offspring,  they  cannot  trans- 
mit directly  to  future  generations.  According  to  Weismann's 
theory  these  characters  are  supposed  to  result  from  fortuitous 
favourable  variations  of  the  germ  plasm  of  the  parents,  which  are 
accumulated  and  intensified  under  the  influence  of  natural 

1  Pp.  142-3.  I  have  perhaps  laid  too  much  stress  upon  this  case  ;  chemotaxis 
may  have  more  to  do  with  it  than  appears  at  first  sight. 


190       OUTLINES   OF   E VOLUTION ABY  BIOLOGY 

selection,  the  welfare  of  the  insect  community  as  a  whole,  rather 
than  that  of  its  constituent  individuals,  being  in  this  case  the 
determining  condition  through  which  natural  selection  operates. 
The  case  of  such  communities,  however,  is  exactly  analogous  to 
that  of  individuals,  except  that  the  single  cells  are  replaced  by 
complete  and  separate  multicellular  units.  The  whole  community 
may  be  looked  upon  as  one  individual  of  a  higher  order,  and  the 
problem  of  the  transference  of  stimuli  from  the  bodies  of  the 
neuters  to  the  germ  cells  of  the  perfect  insects  differs  in  no 
essential  respect  from  that  of  the  transference  of  similar  stimuli 
from  somatic  cells  to  germ  cells  in  an  ordinary  individual.  The 
close  association  in  which  members  of  such  communities  live 
may  be  supposed  to  facilitate  the  transfer  of  such  stimuli  (arising 
from  modification  of  the  somatic  cells  of  the  neuters  in  response  to 
the  environment)  to  the  germ  cells  of  the  males  and  perfect 
females,  without  the  aid  of  material  conductors. 

In  the  present  state  of  our  knowledge,  of  course,  any  suggestions 
which  may  be  put  forward  on  this  subject  must  be  regarded  as  mere 
hypotheses,  incapable  of  demonstration,  and  as  such  they  will 
doubtless  appear  to  many  people  to  be  unwarrantable.  If,  however, 
they  serve  to  show  that  there  is  no  a  priori  reason  for  denying  the 
possibility  of  the  transmission  of  somatogenic  characters  to  the 
germ  cells,  and  thence  to  future  generations,  they  win^se£vg  a 
useful  purpose.  ^  f^)  /X 

We  must,  however,  again  point  out  thavunaer  nonna] 
cumstances,  it  probably  takes  many  generations  before  any 
impression  produced  by  modification  of  the  soma  upon  the  germ 
cells  becomes  so  deeply  ingrained  as  to  find  full  expression  in  the 
offspring  produced  by  their  development.  The  impression,  once 
made,  appears  to  be  equally  difficult  to  remove,  and  hence  has 
probably  arisen  that  conservatism  or  inertia  of  the  germ  cells  to 
which  we  have  already  alluded.  Occasionally,  however,  it  appears 
that  a  suddenly  acquired  somatogenic  character  at  once  makes  a 
deep  impression  upon  the  germ  cells.  Why  this  should  be  so  we  do 
not  know,  but  it  is  perhaps  no  more  remarkable  than  the  well 
known  fact  that  the  germ  cells  themselves  may  occasionally  throw 
aside  their  conservatism  and  give  rise  to  sports  or  mutations.1 

It  is  well  known  that,  apart  altogether  from  the  highly 
specialized  brain  cells,  living  protoplasm  frequently  has  the  power 

1  It  is  possible,  however,  that  the  suddenness  of  a  mutation  is  apparent  rather 
than  real,  at  any  rate  in  some  cases  ;  compare  p.  156. 


MNEMIC   THEOBY  OF  HEREDITY     (gl     191 

^> 

not  only  of  receiving  and  responding  immediately  to  stimuli  of 

various  kinds,  but  also  of  storing  up  impressions  or  engrams  for 
future  use.  This  is  demonstrated  quite  clearly  by  the  phenomena 
known  as  "  after-effects  "  in  various  plants  and  animals.  One  of 
the  best  known  examples  of  such  after-effects  is  seen  in  the  daily 
periodicity  of  plant  growth.  The  light  of  the  sun  acts  as  a 
restraining  or  inhibiting  stimulus  upon  the  rate  of  growth  of 
ordinary  plants.  In  consequence  of  this  the  plant  grows  most 
rapidly  in  the  early  hours  of  the  morning  after  a  prolonged 
exposure  to  darkness,  and  most  slowly  in  the  afternoon,  after 
long  exposure  to  daylight.  It  has  been  shown  that  this  daily 
periodicity,  or  variation  in  rate  of  growth  in  correspondence  with 
the  periodic  variation  in  environment,  is  continued  when  the 
plant  is  kept  in  perpetual  darkness,  and  the  direct  stimulation  of 
changing  environment  therebv  rendered  impossible.  In  other 
words,  the  plant,  though  aapfely  devoid  of  any  nervous  system 
in  the  ordinary  sense  m^t^Herm,  establishes  a  habit,  which  must 
depend  upon  sometJyjgMfcalogous  to  memory  on  its  part. 

The  mnemic  thi^^oT^p^dity_is  based,  as  we  have  seen,  upon  a 
comparison  of  the  pnenomena  of  inheritance  with  those  of  memory. 
The  latter  are  reasonably  explained  Ly^upposing  that  impressions 
received  by  certain  cells  of  the  brain  may  be  stored  up  as 
"  engrams  "  in  these  cells  for  use  on  future  occasions,  when, 
under  appropriate  stimulation,  they  give  rise  to  mental  condi- 
tions corresponding  to  those  produced  by  the  original  stimuli. 
The  stimulus  which  evokes  a  memory  is  commonly  the 
repetition  of  some  stimulus  which  was  originally  associated 
with  the  thing  remembered.  Thus  the  sight  of  a  person  whom 
we  have  not  seen  for  a  long  time,  or  even  of  his  portrait,  will 
evoke  a  whole  train  of  memories  connected  with  that  person. 

The  degree  of  accuracy  with  which  we  remember  any  occurrence 
depends  partly  upon  the  nature  of  the  occurrence  itself  and 
partly  upon  the  frequency  with  which  it  has  been  brought  under 
our  notice.  The  first  time  we  take  a  walk  we  may  have  to  pay 
attention  to  every  turning  and  every  signpost  in  order  to  find  our 
way  at  all,  but  if  we  take  the  same  walk  many  times  we  at  length 
become  so  familiar  with  it  that  we  may  be  thinking  of  other 
things  the  whole  time  and  not  consciously  notice  a  single  land- 
mark from  start  to  finish.  As  Samuel  Butler  has  most  forcibly 
pointed  out,  the  more  perfect  memory  becomes  the  less  is  it 
accompanied  by  consciousness  of  the  things  remembered,  as  when 


192        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

a  skilled  musician,  playing  a  composition  for  perhaps  the 
hundredth  time,  is  quite  unconscious  of  the  individual  actions 
which  he  performs  by  memory  in  striking  the  notes,  while  a 
beginner,  who  has  only  played  the  same  piece  once  or  twice 
before,  may  be  acutely  conscious  of  every  note  which  he  strikes. 
The  development  of  an  individual  organism  from  the  egg  is, 
according  to  the  mnemic  theory,  merely  an  unconscious  repetition, 
by  memory,  of  acts  which  have  been  performed  many  thousands 
of  times  by  its  ancestors,  and,  as  in  the  performance  of  a  piece  of 
music,  each  successive  act  constitutes  the  stimulus  which  calls 
forth  the  next. 

The  fertilized  ovum  may  be  looked  upon  as  being  charged  with 
the  latent  memories  of  past  generations.  The  unconscious  memory 
of  what  it  did  last  time  it  was  a  fertilized  ovum  prompts  it  to  divide 
into  two  cells,  this  re-arrangement  of  its  constituents  supplies 
another  stimulus  which  prompts  it  to  a  further  division,  and  so  on 
through  all  the  stages  of  ontogenetic  development.  In  the  words 
of  Professor  Francis  Darwin,  "the  rhythm  of  ontogeny  is  actually 
and  literally  a  habit." * 

Progressive  evolution  takes  place  owing  to  the  fact  that  each 
successive  generation  may  add  a  little  bit  to  the  record  on  its  own 
account,  this  addition  being  the  result  of  some  new  experience 
due  to  some  difference  in  its  environment  as  compared  with  the 
environment  of  its  predecessors. 

There  are,  then,  two  distinct  sets  of  factors  which  determine 
the  individual  development  of  any  organism,  first}the  inherited 
tendencies,  or  "  engrams,"  and  second^  the  stimuli  provided  by 
the  environment  during  its  own  life-time.  An  egg  placed  under 
very  unusual  conditions,  as,  for  example,  too  low  or  too  high  a 
temperature,  may  be  unable  to  develop  at  all,  but  if  the 
change  of  conditions  be  but  slight  the  organism  may  adapt 
itself,  educate  itself  and  furnish  itself  with  a  new  store  of 
experiences,  which  in  course  of  time  may  be  impressed  upon  the 
germ  plasm  and  thus  added  to  the  record  and  handed  on  to 
future  generations. 

We  have  already  had  occasion  to  notice,  in  the  case  of 
artificially  produced  cyclopean  fish  larvae,  how  an  alteration  of  the 
environment  not  sufficient  altogether  to  prevent  development 
may  so  far  overcome  the  inherited  tendencies  of  the  organism 
as  to  give  rise  to  the  production  of  monstrosities  (p.  157). 

1  Presidential  Address  to  the  British  Association,  Dublin,  1908 


. 
ACTION   OF   THE   ENVIRONMENT  193 

Such  cases  clearly  prove  the  immense  influence  of  the  environ- 
ment in  determining  the  character  of  the  organism.  If  we  admit 
that  inherited  tendencies  also  owe  their  origin  in  the  first  instance 
to  the  action  of  the  environment  on  previous  generations,  it  is 
difficult  to  avoid  the  conclusion  that  every  organism  is  ultimately 
indebted  for  all  its  characters  to  the  action  of  external  stimuli 
upon  responsive  protoplasm. 


r\ 


B. 


CHAPTEK  XIV 

The    Mendelian    experiments    in    hybridization  —  The    doctrine    of    unit 
characters  and  the  purity  of  the  gametes — Galton's  law  of  inheritance. 

WHATEVER  view  we  may  adopt  as  to  the  mechanism  of  heredity 
there  can  be  no  doubt  as  to  the  existence  of  a  material  foundation 
for  the  transmission  of  characters  from  parent  to  offspring.  We 
know  that  the  protoplasm  which  forms  the  material  basis  of 'all 
living  things  is  continuous,  by  means  of  the  germ  cells  or  gametes, 
in  an  unbroken  stream  from  one  generation  to  another.  At  each 
conjugation  of  gametes,  however,  two  such  streams  are  mingled, 
and  the  offspring  receives  part  of  its  initial  stock  of  protoplasm 
from  one  parent  and  part  from  the  other,  each  part  bringing  with 
it  all  the  potentialities  which  may  have  been  derived  from  a  long 
line  of  ancestors.  Bearing  these  facts  in  mind,  we  must  now 
direct  our  attention  to  some  results  which  have  been  obtained 
by  the  "Mendelian"  method  of  attacking  the  problem  ol 
heredity. 

In  the  early  part  of  the  nineteenth  century  much  attention  was 
paid  by  horticulturists  to  experiments  on  the  hybridization  of 
plants.  It  was  found  that,  within  certain  limits,  it  was  possibl< 
to  fertilize  the  flower  of  one  variety  of  plant  with  the  pollen 
of  another  variety,  and  in  this  manner  to  alter  the  character 
of  the  offspring.  Many  crosses  or  hybrids  between  different 
varieties  were  thus  obtained,  differing  in  various  ways  from 
the  parent  plants.  It  was  also  found  that  by  repeatedly  fertilizing 
the  flowers  of  one  variety  with  the  pollen  of  another  the 
descendants  of  the  first  could  be  entirely  changed  into  the  second. 
This  clearly  proved  that  all  the  distinctive  characters  of  the  male 
parent  were  in  some  way  represented  in  the  pollen  grains,  and 
could  be  transmitted  by  these  to  future  generations,  but  it  air 
proved  something  more,  it  proved  that  it  was  possible 
eliminate  the  special  characters  of  the  female  parent,  or  at  lea 
to  prevent  them  from  manifesting  themselves  in  the  offspring. 

As  a  rule  it  is  only  possible  to  bring  such  experiments  to 


MENDEL'S  WORK  ON   HYBRIDIZATION          195 

successful  issue  when  working  with  closely  related  species  or 
varieties.  The  flower  of  one  kind  of  orchid,  for  example,  may 
perhaps  be  fertilized  by  the  pollen  of  a  different  orchid,  but  the 
pollen  of  such  a  plant  as  a  lily  would  probably  not  have  the 
slightest  effect  upon  it. 

The  first  observer  to  throw  a  clear  light  upon  the  meaning  of 
the  remarkable  results  obtained  by  hybridization  was  Gregor 
Johann  Mendel,  a  native  of  Austrian  Silesia,  born  in  1822,  whose 
work  has  recently  attracted  so  much  attention.  As  Abbot  of 
Briinn,  he  was  the  happy  possessor  of  a  garden,  and  presumably 
also  of  that  peace  and  quiet  which  are  so  essential  to  intellectual 
work.  He  was  not  content  with  merely  casual  experiments  ;  he 
ha<J  time  to  think  about  what  he  was  doing  and  he  attacked  the 
problem  in  the  true  scientific  spirit.  Unfortunately  for  science, 
however,  his  seclusion  was  a  little  too  complete.  He  was  content 
to  publish  his  results  in  1865  and  1869  in  the  proceedings  of  a  local 
natural  history  society,  where  they  remained  buried  and  almost 
unnoticed  for  more  than  thirty  years.  Thus,  by  the  irony  of  fate, 
our  own  illustrious  countryman,  Charles  Darwin,  although  a  con- 
temporary of  Mendel,  probably  never  heard  of  those  remarkable 
discoveries  which  bid  fair  to  solve  some  of  the  problems  in  which 
-he  himself  was  so  deeply  interested. 

In  order  to  gain  an  insight  into  the  nature  of  Mendel's  work 

we  cannot  do  better  than  turn  to  his  original  memoir,  entitled 

"  Experiments  in  Plant  Hybridization,"  of  which  an  excellent 

translation  has  been  published  by  Professor  Bateson,1  one  of  the 

eading  exponents  of  what  is  now  termed  Mendelism.    Even  fifty 

years  ago,  experiments  in  hybridization  were,  as  we  have  already 

seen,  no  new  thing.     Mendel  had  many  predecessors  in  this  line 

of   research,   but   it   was   reserved   for  him   to   introduce   exact 

statistical  methods  into  the  work,  and  it  is  to  these  methods  that 

he  owed  his  success.     It  was  already  known  that  hybridization, 

or  the  crossing  of  distinct  species  and  varieties,  might  result 

in  the  production    of   several   distinct  types  of  offspring  from 

the  same   hybrids.     Mendel   was   not   content  with  this  know- 

'edge:    he  proceeded  by  numerous  and   often  repeated  experi- 

onts,  extending   over   several  years,   to  classify   the  offspring 

oduced  by  hybridization,  to  follow  out  their  descendants  from 

neration  to  generation,  and  above  all  to  find  out  the  exact 

Truer ical  proportions  in  which  the  different  types  appeared  in 

1   Vide  Bateson,  "  Mendel's  Principles  of  Heredity  "  (Cambridge,  1909). 

o  2 


196       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

successive  generations.    It  was  the  discovery  of  these  proportions 
that  furnished  the  clue  to  the  mystery. 

Mendel  found  the  material  which  he  required  for  his  experi- 
ments chiefly  in  the  numerous  garden  varieties  of  the  common 
edible  pea.  How  these  varieties  first  originated  we  do  not  know. 
Possibly  they  arose  as  sudden  and  apparently  spontaneous 
variations,  or  mutations.  No  less  than  twenty-two  such  varieties 
were  chosen  for  experiment. 

The  next  thing  was  to  select  certain  differentiating  characters 
upon  which  to  concentrate  attention,  and  this  is  a  very  important 
point.  Of  course,  all  the  varieties  agreed  with  one  another  in 
most  of  their  characters ;  in  other  words,  they  were  all  edible 
peas  exhibiting  the  specific  characters  of  the  plant  known  to 
botanists  as  Pisum  sativum,  but  they  differed  in  numerous  minor 
features.  Of  these  differentiating  characters  Mendel  selected 
seven  for  the  purposes  of  his  experiments,  amongst  which  I  need 
mention  only  two  :  (1)  the  form  of  the  ripe  seed,  which,  roughly 
speaking,  may  be  either  round  and  smooth  or  angular  and 
wrinkled  ;  and  (2)  the  difference  in  colour  of  the  seed-contents  or 
cotyledons,  which  may  be  either  yellow  or  green  and  which  usually 
determine  the  colour  of  the  seed  as  a  whole. 

The  two  differentiating  characters  of  each  pair  were  artificially 
united  by  cross-fertilization,  the  flowers  of  a  round-seeded  pea 
being  fertilized  by  the  pollen  of  a  wrinkled-seeded  pea,  those  of  a 
green-seeded  pea  by  the  pollen  of  a  yellow-seeded  pea,  and  so  on. 

In  this  way  a  number  of  hybrid  plants,  represented  in  the  first 
instance  by  seeds,  were  obtained.1  These  hybrids  constituted 
what  modern  writers  term  the  Fl  or  "first  filial"  generation, 
and  the  curious  fact  was  observed  that  every  hybrid  closely 
resembled  one  of  the  two  parents,  instead  of  being,  as  is  frequently 
the  case  in  other  hybrids,  intermediate  in  character  between 
them.  Further  experiments  with  the  offspring  of  the  hybrids, 
however,  showed  that,  although  only  one  of  the  two  contrasted 
characters  manifested  itself  in  the  hybrid,  the  other  was  there 
also  in  a  latent  or  dormant  condition,  and  appeared  again  in 

1  It  must  be  remembered  that  the  pea-seed  consists  chiefly  of  the  young  plant 
in  a  dormant  condition,  and  that  its  characters  are  therefore,  with  the  exception 
of  those  appertaining  to  the  seed-coat,  which  are  not  considered  in  this 
chapter,  those  of  the  sporophyte  generation  which  follows  the  conjugation  of  the 
gametes.  The  fact  that  the  complete  life-cycle  of  the  plant  really  includes  two 
generations,  a  well  developed  sporophyte  and  a  vestigial  gametophyte,  does  not 
affect  the  Mendelian  conclusions. 


HYBRIDIZATION   IN   PEAS  197 

subsequent  generations.  The  character  which  appears  in  the 
hybrid  is  said  to  be  dominant,  while  that  which  is  suppressed  is 
said  to  be  recessive.  In  the  case  of  these  peas,  then,  one  character 
is  always  dominant  over  the  other  in  the  hybrid,  and,  moreover, 
it  is  always  the  same  character,  and  it  does  not  matter  whether 
it  is  derived  from  the  male  or  from  the  female  parent.  Thus  the 
round  form  of  seed  is  dominant  over  the  wrinkled,  the  yellow 
colour  of  the  seed-contents  over  the  green,  and  so  on. 

Having  obtained  the  hybrids,  the  next  step  was  to  follow  the 
history  of  the  offspring  throughout  successive  generations.  For 
this  purpose  the  flowers  of  the  plants  raised  from  the  hybrid 
seeds  were  allowed  to  fertilize  themselves  with  their  own  pollen, 
no  further  crossing  being  permitted.  The  seeds  thus  obtained 
(constituting  the  F2  or  "  second  filial "  generation)  were  now 
again  apparently  of  two  kinds,  resembling  the  two  original  parent 
forms  from  which  the  hybrid  was  produced.  Moreover,  these  two 
kinds  occurred  in  definite  proportions,  three  of  the  dominant  to 
one  of  the  recessive — three  round  seeds  to  one  wrinkled,  three 
yellow  to  one  green,  and  so  on.  Of  course,  these  proportions  are 
only  averages,  and,  in  order  to  eliminate  errors  of  chance,  large 
numbers  of  observations  must  be  made — the  greater  the  number 
the  more  reliable  the  result.  To  take  an  actual  example  from 
Mendel's  work,  7,324  seeds  were  obtained  in  the  second  trial  year 
from  the  hybrids  between  round  and  wrinkled.  Of  this  number 
5,474  were  found  to  be  round,  and  1,850  wrinkled — showing  the 
ratio  of  2'96  to  1.  Again,  out  of  8,023  seeds  produced  by  the 
hybrids  between  green  and  yellow,  6,022  were  yellow  and  2,001 
green,  the  ratio  being  3*01  to  1.  These  experiments  have  often 
been  repeated  during  recent  years,  especially  by  Mr.  A.  D. 
Darbishire,1  and  it  is  found  that  the  average  ratio  of  three  to  one 
is  always  maintained. 

The  significance  of  this  proportion  is  not  at  first  sight  obvious. 
It  is  necessary  to  continue  the  experiment  for  at  least  another 
generation  in  order  to  gain  further  insight  into  the  matter.  We 
have  as  the  result  of  the  self-fertilization  of  our  hybrids 
apparently  only  two  kinds  of  seeds  or  plants,  which  we  may 
call  D,  showing  the  dominant  character,  and  E,  showing  the 
recessive  character,  in  the  proportion  of  3  D  to  1  E.  If  these 

1  Some  beautiful  illustrations  of  Mendelian  results  in  both  plants  and  animate 
are  given  in  Mr.  Darbishire's  recent  work  "  Breeding  and  the  Mendelian  Discovery  " 
(Cassell  &  Co.,  Ld.,  1911). 


198        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


plants  are  again  cultivated  and  allowed  to  fertilize  themselves  we 
find  that  every  one  of  the  R's  breeds  true,  and  no  matter  how 
many  generations  we  raise  they  will  always  remain  of  the 
recessive  type.  These  are  now  called  "extracted  recessives." 
When,  however,  we  cultivate  the  plants  showing  the  dominant 
character  in  the  same  way  we  soon  find  that,  though  similar 
to  one  another  externally,  they  are  not  in  reality  all  alike, 
for  one-third  of  them  will  yield  nothing  but  D's,  while  the 
remaining  two-thirds  will  yield  D's  and  R's  in  the  same 
proportion  of  3  to  1  as  the  original  hybrid.  Moreover,  the 
D's  obtained  from  the  one-third  will  continue  to  breed  true 
from  generation  to  generation,  and  do  what  we  will  we  can 
never  get  an  R  out  of  them  again.  They  are  called  "  extracted 
dominants." 

It  is  obvious  then,  from  these  experiments,  that  the  apparent 
D's  are  not  all  true  D's,  but  that  two-thirds  of  them  contain  the 
recessive  character  in  a  latent  or  dormant  condition,  and  are, 
therefore,  still  hybrid  in  composition.  If  we  indicate  those  which 
we  can  thus  prove  by  breeding  to  contain  the  recessive  character 
by  the  letters  D  (R),  we  can  sum  the  whole  story  up  in  the  following 
simple  diagram : — 

D  x   R  (Original  Parents) 

Generation 


D     DP     D 

Extracted  dominants 


D    D®  D®  R       D  D®  D®  R 
FIG.  79. — Monohybridism. 


R      R     R      R  -  -  -  F3 


Extracted  recess 


Thus  we  see  that  from  generation  to  generation  of  the 
offspring  of  the  hybrid  there  goes  on  a  constant  sorting  out  into 
three  categories,  the  two  original  parental  types  and  the  hybrid 
form  being  always  produced  in  definite  proportions.  If  we 
assume  that  each  kind  produces  on  an  average  an  equal  number 
of  offspring,  we  shall  see  from  the  diagram  that  in  the  course  of  a 
few  generations  of  undisturbed  reproduction  by  far  the  greater 
number  of  the  plants  will  have  finally  reverted,  in  equal  propor- 
tions, to  one  or  other  of  the  original  types,  but  a  certain  number 


EXPLANATION   OF  MENDELIAN  RESULTS      199 

will  always  remain  in  the  hybrid  condition  and  will  continue 
to  split  up  as  before  in  the  characteristic  Mendelian  proportions. 
We  see,  then,  from  Mendel's  experiments,  that  in  the  case  of 
peas,  where  one  of  the  two  contrasted  characters  is  dominant 
over  the  other,  the  offspring  of  the  first  hybrid  appear  in  the 
proportion  of  three  apparent  dominants  to  one  recessive,  but  that 
further  analysis  shows  that  the  real  proportion  is 

1  D  :  2  D  (K)  :  1  K.1 

This  proportion  has  since  been  observed  not  only  in  peas  but 
in  a  large  number  of  other  cases,  including  animals  as  well  as 
plants.  It  is  evidently  a  phenomenon  of  very  common 
occurrence,  though  we  cannot  as  yet  say  that  it  occurs 
universally  whenever  two  forms  with  contrasted  characters  are 
crossed.  Moreover,  as  we  have  already  noticed,  the  phenomenon 
of  dominance  is  not  always  shown,  and  the  hybrid  may  exhibit 
a  character  intermediate  between  those  of  the  two  parents  or 
different  from  either.  The  occurrence  of  Mendelian  propor- 
tions, however,  is  sufficiently  frequent  to  demand  explanation 
and  this  explanation  we  must  now  seek. 

Suppose  we  take  a  large  number  of  black  and  an  equal  number 
of  white  counters,  all  alike  in  shape,  size  and  weight,  and  after 
shaking  them  up  thoroughly  in  a  bag,  draw  them  out  two  at  a 
time,  and  one  on  top  of  the  other,  without  looking.  Each  pair 
that  we  draw  out  may  consist  of  white  over  white,  black  over  white, 
white  over  black,  or  black  over  black.  If  we  draw  out  a  sufficiently 
large  number  of  pairs  entirely  at  random  in  this  way  and  then  count 
them,  we  shall  find  that  they  occur  in  the  proportion  : — 1  W  W  : 
1  B  W :  1  W  B :  1  B  B,  or,  since  B  W  and  W  B  may  be  looked 
upon  as  the  same,  1  W  W :  2  W  B  :  1  B  B,  or  one  all  white,  two 
white  and  black,  and  one  all  black ;  this  being,  of  course,  only 
what  is  to  be  expected  in  accordance  with  the  mathematical  law 
of  probability.  This  is  also  the  same  as  the  simple  Mendelian 
proportion,  and  at  once  suggests  that  the  latter  may,  perhaps,  be 
explained  in  a  similar  way  as  the  result  of  random  union  of 
characters  in  accordance  with  the  laws  of  chance. 

We  know  that  each  individual  plant  or  animal  produced  by. 
sexual  reproduction  is  formed  by  the  union  of  two  germ  cells  or 
gametes.  Now  these  gametes  are  formed  in  very  large  numbers 

l  This  is  sometimes  written  more  fully  1  D  D  :  2  D  (R)  :  1  R  R.  The  reason 
for  so  doing  will  be  obvious  from  what  follows. 


200        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

in  the  parent  organisms,  and  if  we  suppose  that  each  one  contains 
only  one  of  the  two  contrasted  characters  with  which  we  are 
experimenting,  and  if  we  further  suppose  that  the  two  kinds  are 
produced  in  equal  proportions  and  that  they  unite  at  random, 
then,  in  accordance  with  the  law  of  chance,  we  should  expect 
to  get  in  the  offspring  the  proportion  of  one  with  the  one 
character,  two  with  both  characters  and  one  with  the  other 
character,  exactly  as  Mendel  found  in  his  experiments.1  There 
seems  to  be  no  other  possible  explanation  of  the  Mendelian 
phenomena. 

The  conclusions  to  be  drawn  from  these  results  are  of 
fundamental  importance.  In  the  first  place,  we  learn  that  small 
individual  characters  may  be  separately  represented  in  the 
germ  cells  and  separately  transmitted  from  parent  to  offspring. 
This  indicates  that  the  entire  organism  may  be  built  up  of  a 
number  of  "unit  characters,"  and  if  we  can  once  establish 
the  general  occurrence  of  such  unit  characters  we  shall  have 
taken  a  long  stride  towards  the  understanding  of  the  laws  of 
heredity. 

In  the  second  place,  we  may  conclude  from  these  experiments 
that,  as  regards  the  unit  characters  with  which  we  are  dealing, 
we  have  a  complete  segregation  amongst  the  germ  cells  or 
gametes,  each  of  which  carries  only  one  of  each  pair  of  contrasted 
characters  (technically  termed  allelomorphs),  and  in  this  respect 
it  makes  no  difference  whether  the  germ  cell  be  male  or  female. 
Thus  we  arrive  at  what  is  sometimes  termed  the  doctrine  of  the 
"purity  of  the  gametes" — purity,  that  is  to  say,  with  regard  to 
each  character  of  any  such  contrasted  pair,  and  without  reference 
to  the  other  innumerable  characters  which  must  be  represented 
in  the  germ  cells  in  order  that  the  whole  complex  structure  of  the 
organism  may  be  developed  from  them. 

We  may  now  examine  an  example  of  a  class  of  practical 
problems  which  may  be  solved  by  the  application  of  such  simple 
Mendelian  principles. 

Fowl-fanciers  are  well  acquainted  with  a  particular  breed  of 
fowl  known  as  the  blue  Andalusian.  It  has  long  been  known 
that  this  kind  of  fowl  cannot  be  made  to  breed  true.  Some  of 

1  A  little  consideration  will  show  that  if  the  contrasted  characters  are  repre- 
sented in  equal  proportions  of  both  male  and  female  gametes,  the  facts  that  a 
male  must  always  unite  with  a  female,  and  that  the  actual  number  of  sperm  cells 
produced  is  usually  very  much  larger  than  the  number  of  ova,  will  not  affect  the 
result. 


HYBRIDIZATION   IN   FOWLS  201 

the  offspring  will  be  black  and  others  white  with  black  splashes, 
the  remainder  being  like  the  parents.  It  has  been  shown, 
moreover,  that  the  three  kinds  of  chicken  occur,  on  an  average,  in 
definite  proportions,  a  quarter  being  black,  a  half  blue,  and  a 
quarter  white  splashed  with  black.  Here  we  have  the  familiar 
Mendelian  proportion  1:2:1,  suggesting  that  the  blue  Andalusian 
is  really  a  hybrid,  and  that  the  so-called  "  wasters  "  are  the  parent 
forms.  It  is  easy  to  prove  that  this  is  the  case,  for  if  the  two 
kinds  of  waster  are  mated  we  invariably  get  the  blue  Andalusian 
again.  The  old  idea  of  the  so-called  practical  breeder  would  have 
been  to  go  on  destroying  the  wasters  and  carefully  selecting  the 
blues  in  order  to  maintain  the  "  purity  of  the  breed."  We  now 
know,  however,  that  there  is  no  such  thing  as  a  pure  blue  Andalu- 
sian breed  ;  the  blue  is  really  a  hybrid,  and  you  can  get  more  blues 
by  mating  the  wasters  than  by  breeding  from  the  blues 
themselves.  This  case  is  also  interesting  as  affording  an  example 
of  a  hybrid  which  differs  in  character  from  either  of  the  parent 
forms. 

If  only  a  single  pair  of  alternative  characters  or  allelom orpins  is 
dealt  with  in  the  experiment  the  case  is  termed  one^oTmono- 
hybridism,  and  such  cases  yield  the  proportion  1  :  2  :  1  (or 
apparently  1  :  3  in  cases  of  dominance)  amongst  the  offspring  of 
the  hybrid,  i.e.  in  the  F2  generation.  It  will  frequently  happen, 
however,  that  the  two  varieties  which  are  united  in  the  formation 
of  the  hybrid  will  differ  from  one  another  as  regards  more  than 
one  pair  of  contrasted  characters.  We  may  illustrate  this  by  a 
case  of  dihybridism,  in  which  two  pairs  of  contrasted  characters 
are  involved.  The  case  selected  is  one  which  shows  us  how, 
under  certain  circumstances,  we  can  obtain  a  hybrid  exhibiting 
an  entirely  new  combination  of  characters,  which  in  spite  of  its 
hybrid  nature  will  continue  to  breed  true. 

Suppose  a  gardener  had  only  two  kinds  of  peas,  one  with  round 
green  seeds  and  the  other  with  wrinkled  yellow  seeds,  and  that  he 
wished  to  obtain  peas  with  wrinkled  green  seeds.  Mendel  has 
shown  us  how  he  may  get  what  he  desires,  both  surely  and 
speedily,  from  the  material  already  in  his  possession. 

The  necessary  procedure  will  be  evident  from  the  following 
scheme : — 

Let  R  =  round,  W  =  wrinkled,  Y  =  yellow,  and  G  =  green. 
We  know  from  previous  experiments  that  R  is  dominant  to  W, 
and  Y  to  G. 


202       OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

E  G  crossed  with  W  Y  gives  the  hybrid,E'(G)  (W)  Y,  which,  owing 
to  the  dominance  of  E  and  Y,  will  appear  in  the  form  of  round 
yellow  seeds  (the  brackets  around  the  G  and  W  indicating  the 
recessive  nature  of  green  and  wrinkled).  That  is  not  what  our 
gardener  wants,  and  if  he  knew  no  better  he  would  naturally  be 


FIG.  80.— An  Example  of  Dihybridism  in  Peas. 

A,  wrinkled  yellow  peas ;  B,  round  green  peas ;  C,  hybrid  peas  (Fi  generation,  round 
and  yellow  in  appearance)  produced  by  crossing  A  and  B ;  D,  plant  of  the  F1 
generation,  with  seeds  of  the  F2  generation,  produced  by  self-fertilization  and  of  the 
four  different  kinds,  round  yellow,  round  green,  wrinkled  yellow,  and  wrinkled  green, 
lying  in  the  pods.  (Photographed  from  a  preparation  by  Mr.  Darbishire,  who  has 
reproduced  the  same  subject  in  colours  in  his  work  on  "  Breeding  and  the  Mendelian 
Discovery  "). 

much  disappointed  with  the  result.  If,  however,  the  hybrids  be 
allowed  to  fertilize  themselves  and  reproduce,  then  their  off- 
spring will  appear  in  the  apparent  proportions  :— 

9EY  :  3EG  :  3  WY  :  1  W  G, 

no  account  being  taken  of  the  non-apparent  recessive  characters. 
The  meaning  of  the  proportion  9:3:3:1  exhibited  in  this 


DIHYBRIDISM 


203 


case  will  be  readily  understood  from  the   accompanying  table 
(Fig.  81)  :- 

T       — — - 

R  Y 


RG 


WY 


WG 


R  Y  . 

R(G) 

(W)  Y 

(W)(G) 

R  Y 

R  Y 

R  Y 

R  Y 

RY 

RG 

(W)  Y*  " 

(W)  G- 

R(G) 

RG 

R(G) 

RG 

(/ 
R  Y 

R(G) 

WY 

W(G)    0 

(W)Y 

(W)Y 

W  Y 

WY 

RY/ 

RG 

W  Y      3 

W  G. 

(W>(G) 

(W)G 

W(G) 

W  G 

ont 


IG.  81. — Dihybridism. 


The  constitution  of  the  hybrid  (in  so  far  as  the  characters  in 
question  are  concerned)  is  K  (G)  (W)  Y.  Each  of  its  germ  cells, 
however,  contains  one  character,  and  one  only,  from  each  con- 
trasted pair.  Each  may,  therefore,  contain  E  Y,  K  G,  W  Y  or 
W  G.  These  different  kinds  of  germ  cells  will  occur  on  the 
average  in  equal  numbers,  and  on  self-fertilization  of  the  hybrid 
flowers  they  will  unite  in  pairs  at  random.  The  possible 
ways  in  which  such  union  may  take  place  are  shown  in  the  table, 
and  if  allowance  be  made  for  the  phenomenon  of  dominance,  in 
accordance  with  which  G  disappears  from  view  whenever  it  meets 
Y,  and  W  whenever  it  meets  R  (as  indicated  by  the  brackets  in 
the  table),  we  g^t  the  apparent  proportion  of  9  round  yellow 
seeds,  3  round  green  seeds,  3  wrinkled  yellow  seeds,  and  1  wrinkled 
green  seed.  The  number  of  wrinkled  green  seeds  will  be  small  at 
first,  but  they  will  breed  true,  containing  as  they  do  only  the 
extracted  recessive  characters,  for  if  a  dominant  character  were 
present  it  would  necessarily  show  itself. 

Thus  we  see  that  one  way  of  producing  new  forms  of  plants 
and  animals  is  by  the  artificial  combination  of  characters 
which  already  exist  in  different  varieties.  These  elementary  or 
unit  characters  can  be  brought  together  by  the  process  of  hy- 
bridization, and  new  organisms  produced  in  somewhat  the  same 


204        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

way  as  that  in  which  the  chemist  is  able  to  produce  new  compounds 
by  analysis  and  synthesis  from  other  substances. 

Probably  no  man  has  made  more  successful  use  of  the  process 
of  hybridization  in  the  production  of  new  and  valuable  forms  of 
plant-life  than  Luther  Burbank,  at  his  celebrated  Californian 
nurseries.  Mr.  Burbank  himself  is  unfortunately  not  a  writer, 
and  for  a  scientific  account  of  his  work  we  are  indebted  to 
Professor  Hugo  de  Vries,  who  in  his  book  on  "  Plant  Breeding"1 
describes  what  he  himself  saw  and  heard  during  his  visits  to 
Mr.  Burbank's  farms.  Some  idea  of  the  commercial  value  of 
Mr.  Burbank's  work  may  be  formed  from  the  fact  that  special 
companies  have  been  formed  for  the  propagation  and  sale  of 
some  of  these  wonderful  hybrids.  White  blackberries,  stoneless 
prunes,  "  plumcots  "  and  thornless  cacti  are  only  some  amongst 
the  many  novelties  which  he  has  produced  by  crossing  different 
varieties  by  hybridization  and  thus  combining  two  or  more 
desirable  qualities  in  one  plant. 

TakeT  for  instance,  the  case  of  the  stoneless_prune.  It  had 
somehow  or  other  come  to  the  knowledgq^j:  Mr.  Burbank  that 
about  200  years  ago  there  existed  in  France  a  plum  known  as  the 
"  Prune  sans  noyau."  He  succeeded  in  obtaining  specimens  of 
this  fruit,  but  it  proved  to  be  of  little  or  no  commercial  value 
owing  to  its  poor  quality.  Its  one  valuable  feature  was  its  stone- 
lessness,  and  Burbank  set  to  work  to  transfer  this  character,  by 
hybridization,  to  a  variety  of  good  quality.  He  succeeded,  and 
there  appears  to  be  no  reason  why  the  stoneless  character  should 
not  be  similarly  implanted  upon  all  the  different  varieties  of 
plums  now  in  cultivation. 

Similarly,  the  "  plumcots  "  are  hybrids  between  plums  and 
apricots,  which  Professor  de  Vries  speaks  of  as  "  most  delicious 
and  beautiful  fruits." 

It  must  be  admitted  that  Burbank's  work  is  practical  rather 
than  scientific.  He  apparently  pays  no  attention  to  the  law  of 
Mendel  in  his  operations,  and  just  goes  on  hybridizing  until  he 
gets  what  he  wants.  In  this  process  enormous  numbers  of  plants 
which  do  not  fulfil  his  requirements  have  to  be  destroyed  for 
every  one  that  is  worth  preserving. 

He  is  not  necessarily  concerned  with  the  production  of  pure 
breeds  ;  of  forms,  that  is  to  say,  which  will  breed  true  from  seed, 

1  Hugo  de  Vries,  "  Plant  Breeding  "  (London,  Kegan  Paul,  Trencli,  Triibner  &  Co., 
Ld.,  1907^ 


PEACTICAL   APPLICATIONS  205 

or,  in  other  words,  of  forms  which  will  hand  on  their  desirable 
qualities  to  future  generations  by  heredity.  This  does  not  matter 
in  the  case  of  fruit  trees  and  other  plants  which  can  be  propa- 
gated by  buds  independently  of  sexual  reproduction.  As  a 
general  rule,  a  fruit  tree  cannot  be  relied  upon  to  come  true 
from  seed,  the  characters  which  it  has  received  from  different 
ancestors  not  being  permanently  combined,  but  separating  out 
and  undergoing  fresh  combinations  in  the  sexual  process,  pro- 
bably in  accordance  with  Mendelian  principles.  The  seedlings 
will  therefore  be  "  degenerate,"  as  horticulturists  say,  and  will  no 
longer  exhibit  those  valuable  qualities  which  depend  upon  the 
confluence  in  one  individual  of  particular  lines  of  ancestry. 

The  majority  of  Burbank's  productions  could  not  survive  in  a 
state  of  nature  at  all ;  they  are  essentially  artificial  and  have  to 
be  artificially  propagated  by  means  of  buds  or  cuttings.  In  this 
respect  they  are  quite  different  from  the  pure  races  which  it  is 
possible  to  produce  by  hybridization  carried  out  in  accordance 
with  Mendelian  principles,  in  which  new  and  permanent  com- 
binations of  unit  clAacters,  capable  of  being  transmitted  by 
heredity,  are  effected. 

Professor  Bateson,  Professor  Biffen,  Mr.  Hurst  and  others  have 
lately  done  much  to  demonstrate  the  possibilities  of  progress  in 
this  direction,  and  the  value  which  the  application  of  the 
Mendelian  principles  of  heredity  must  have  from  the  economic 
point  of  view.  We  know  now  that  such  cereals  as  wheat  and  barley 
obey  the  Mendelian  laws  of  hybridization  as  regards  various 
important  characters.  So  also  do  horses  as  regards  the  colour  of 
their  coats,  and  human  beings  as  regards  the  colour  of  their  eyes, 
and  in  some  other  respects,  especially  as  regards  the  inheritance 
of  certain  diseases.  In  short,  it  appears  certain  that  Mendelian 
principles  have  a  very  general  application,  and  the  economic 
value  of  the  knowledge  of  such  principles  must  be  enormous. 
Professor  Biffen  has  shown,  for  example,  that  susceptibility  and 
insusceptibility  to  that  destructive  disease  in  wheat  known  as 
"  rust,"  behave  as  Mendelian  characters,  and  that  it  is  possible 
by  a  very  simple  process  of  hybridization  to  confer  immunity 
from  this  disease  upon  naturally  susceptible  varieties. 

From  the  theoretical  point  of  view  the  great  value  of  the 
Mendelian  experiments  lies  in  the  possibilities  which  they  present, 
at  any  rate  in  certain  cases,  of  analyzing  the  constitution  of  the 
hereditary  substance  (germ  plasm),  and  thereby  gaining  some 


206        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

insight  into  the  real  nature  of  heredity.  We  may  regard  the 
existence  of  "  unit  characters  "  as  being  conclusively  demonstrated 
by  these  experiments,  and  the  fact  that  we  are  able  to  interchange 
these  characters  and  to  add  and  subtract  particular  characters  to 
and  from  the  organism  seems  to  indicate  very  clearly  that  the 
germ  plasm  must  contain  material  primordia  or  "  factors,"  as  thety 
are  often  called,  which  are  responsible  for  the  development  of 
these  characters. 

In  so  far  as  they  demonstrate  the  existence  of  such  factors — 
which  evidently  correspond  very  closely  with  Weismann's  hypo- 
thetical determinants — the  Mendelian  experiments  may  be  taken 
as  affording  confirmation  of  Weismann's  theory  of  the  constitu- 
tion of  the  germ  plasm.  Their  results  also  harmonize  very  well 
with  those  of  recent  cytological  investigations.  We  have  seen 
that  there  is  strong  evidence  for  regarding  the  chromatin 
substance  of  the  nucleus  as  the  material  basis  of  heredity.  We 
have  further  seen  that  this  chromatin  is  very  complex  in 
structure  and  that  the  chromosomes,  or  units  of  the  highest  order 
of  which  it  is  composed,  can  in  many  case^be  optically  resolved 
into  chromomeres  or  units  of  the  next  lower  order.  According 
to  Weismann's  theory  these  chromomeres  or  ids  are  in  their 
turn  made  up  of  the  determinants.  Moreover  the  associa- 
tions and  redistributions  of  chromosomes  which  take  place 
in  the  process  of  "  reduction  "  and  in  the  conjugation  of  the 
germ  cells  seem  to  afford  ample  opportunity  for  those  permuta- 
tions and  combinations  of  factors  the  occurrence  of  which  is  demon- 
strated by  the  Mendelian  experiments.  As  Professor  Farmer 
observes,  "  The  facts  of  meiosis l  are  seenjto  fall  completely  into 
line  with  conclusions  drawn  from  experiments  on  breeding  as  far 
as  the  numerical  distribution  of  character  s""is**concerned."  2 

The  doctrine  of  the  "purity  of  the  gametes"  teaches  us  that 
any  given  mature  germ  cell  contains  only  one  member  of  any 
given  pair  of  alternative  factors  or  primordia  (allelomorphs). 
This  may  be  accounted  for  by  the  pairing  of  the  chromosomes 
and  the  subsequent  halving  of  their  number  which  take  place  at 
some  period  or  other  prior  to  the  formation  of  the  gametes. 
There  is  good  reason  to  believe  that  each  member  of  such  a  pair 
of  chromosomes  is  homologous  with  or  morphologically  equivalent 

1  I.e.  the  phenomena  accompanying  the  reduction  in  the  number  of  chromosomes 
which  takes  place  periodically  in  all  typical  organisms  (vide  Chapter  X.). 

2  Croonian  Lecture.     Proceedings  of  the  Royal  Society,  B,  Vol.  79,  1907. 


HOMOZYGOTE  AND  HETEEOZYGOTE     207 

to  the  other,  the  difference  between  them  lying  in  the  fact  that 
one  is  paternal  and  the  other  maternal  in  origin.1  Eeduction  (or 
halving  of  the  total  number  of  chromosomes)  is  accomplished  by 
the  distribution  of  the  two  members  of  each  pair  to  different 
daughter  cells.  Hence  if  the  paternal  and  maternal  chromosomes 
contain  different  alternative  -factors  (or  determinants),  derived 
from  different  parents,  so  also  will  the  gametes  when  these  are 
formed. 

Suppose  we  represent  the  two  members  of  any  pair  of  factors 
by  the  letters  a  and  6,  then  any  one  gamete  may  contain  either 
a  or  b,  but  not  both.  When  the  gametes  unite  in  conjugation  the 
zygote  will  again  have  both  maternal  and  paternal  chromosomes 
and  there  will  be  three  possibilities  as  to  its  constitution  in 
regard  to  the  characters  in  question.  If  a  gamete  containing  a 
happens  to  unite  with  another  containing  a  the  zygote  will 
contain  the  pair  a  a,  and  will  be  termed  in  Mendelian  phraseology 
a  ';  homozygote."  2  If  a  gamete  containing  b  happens  to  unite 
with  another  containing  b  the  zygote  will  contain  the  pair  b  b  and 
will  again  be  a  homozygote.  If,  on  the  other  hand,  a  gamete 
containing  a  unites  with  another  containing  b  the  zygote  will 
contain  the  pair  a  b,  and  such  a  zygote  is  termed  a  "  heterozygote  " 
or  hybrid. 

In  accordance  with  what  is  termed  the  "  presence  or  absence 
hypothesis  "  one  of  the  two  "  factors"  in  an  allelomorphic  pair  may 
be  merely  negative  in  character ;  i.e.  it  may  have  no  real  existence 
as  a  material  primordium  or  determinant.  In  the  words  of 
Professor  Bateson,  "All  observations  point  to  a  conclusion  of  great 
importance,  namely  that  a  dominant  character  is  the  condition 
due  to  the  presence  of  a  definite  factor,  while  the  corresponding 
recessive  owes  its  condition  to  the  absence  of  the  same  factor.  This 
generalization,  which  so  far  as  we  yet  see,  is  applicable  throughout 
the  whole  range  of  Mendelian  phenomena,  renders  invaluable 
assistance  in  the  interpretation  of  the  phenomena  of  Heredity. 
The  green  pea,  for  instance,  owes  its  recessive  greenness  to  the 
absence  of  the  factor  which,  if  present,  would  turn  the  colouring 
matter  yellow,  and  so  forth."  3 

It  is  obvious  that  if,  in  our  general  formula,  we  take  a  to 
represent  the  dominant  factor  a-nd  b  the  recessive,  we  might 

1  Compare  Chapter  X. 

2  This  term  has  been  extended  to  the  fully  developed  organism  which  arises  from 
the  zygote. 

8  Bateson,  "  Mendel's  Principles  of  Heredity  "  (Cambridge,  1909),  pp.  53—54. 


208       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

substitute  not  a  for  b  without  altering  the  result,  and  the  differences 
between  the  paternal  and  maternal  chromosomes  of  any  given  pair 
may  depend  merely  upon  the  absence  from  one  or  the  other  of 
one  or  more  factors  which  are  present  in  its  mate. 

Mendelian  phenomena  are  often  greatly  complicated  and 
rendered  very  difficult  of  interpretation  by  the  fact  that  the 
activity  of  a  given  factor,  or  its  power  to  influence  the  development 
of  the  zygote,  may  be  dependent  upon  the  presence  of  another 
factor  belonging  to  a  different  "  allelomorphic  pair."  In  other 
words,  a  given  character  may  depend  not  merely  upon  the 
presence  of  a  single  factor  in  the  zygote  but  upon  the  co-operation 
of  two  or  more  factors,  and  if  these  factors  happen  to  be  separated 
in  the  process  of  reduction  and  do  not  happen  to  come 
together  again  in  the  union  of  the  gametes  to  form  the  zygote, 
then  the  character  in  question *  will  not  appear  in  the  organism 
developed  from  the  zygote.  In  this  way  it  may  happen  that  a 
character  which  was  present  in  the  ancestors  of  a  particular 
organism  may  disappear  for  many  generations  and  then  suddenly 
re-appear  as  the  result  of  some  cross  or  hybridization  in  which 
the  necessary  factors  happen  to  be  brought  together  again  in  the 
zygote.  In  this  way  are  explained  those  cases  of  "  reversion  "  to 
ancestral  types  which  Darwin  found  to  occur  so  frequently  as  the 
result  of  cross-breeding. 

One  of  the  most  thoroughly  investigated  cases  of  this  kind 2  is 
that  of  certain  white  sweet  peas  belonging  to  the  variety  known 
to  horticulturists  as  "Emily  Henderson."  Plants  of  this  variety 
are  not  really  all  alike  but  differ  from  one  another  in  the  shape  of 
their  pollen  grains,  and  when  the  two  kinds,  distinguishable  only 
in  this  way,  are  crossed,  the  offspring  are  frequently  found  to 
possess  purple  flowers  resembling  those  of  the  wild  Sicilian  pea 
from  which  our  cultivated  varieties  have  been  derived.  It  is  not 
necessary  to  go  into  the  somewhat  complicated  analysis  of  this 
case  in  any  detail,  but  it  has  been  demonstrated  by  the  researches 
of  Professor  Bateson  and  Professor  Punnett  that  the  re-appearance 
of  the  lost  colour  in  the  hybrid  is  due  to  the  re-union  in  the  zygote 
of  certain  factors  which  had  become  separated  at  some  period  or 
other  in  the  ancestral  history  of  the  white-flowered  parents. 

The  existence  of  interactions  of  this  kind  between  the  different 
factors  or  determinants  in  the  germ  plasm,  giving  rise  often  to 

1  Professor  Bateson  terms  such  characters  "  compound  characters." 

2  Bateson,  op.  cit.,  pp.  89  et  seg. 


G ALTON'S   LAW   OF   INHERITANCE  209 

very  complex  results  as  regards  the  characters  of  the  offspring,  is 
a  sufficient  explanation  of  the  fact  that  many  cases  of  hybridism 
are  at  present  incapable  of  interpretation  in  terms  of  the 
Mendelian  theory,  and  such  cases  cannot  be  legitimately  used  as 
arguments  against  the  validity  of  that  theory.  It  is  obviously 
impossible  to  explain  the  results  of  any  particular  cross  until  we 
have  some  knowledge,  not  only  of  the  extremely  complex  con- 
stitution of  the  parental  germ  plasm  on  both  sides,  but  also  of 
the  way  in  which  the  factors  which  meet  in  the  zygote  may  /be 
expected  to  influence  one  another. 

Mendelian  phenomena,  of  course,  can  occur  only  as  a  result 
of  crossing  or  hybridization  at  some  stage  or  other  of  the 
ancestral  history,  and  it  does  not  seem  likely  that  such  crossing 
has  had  any  very  great  influence  upon  the  evolution  of  plants  and 
animals  in  a  state  of  nature.  In  cases  of  monohybridism,  as  we 
have  seen  above,  the  hybrid  form  in  the  course  of  a  few  generations 
would  probably  be  quite  swamped  by  the  numerical  preponderance 
of  the  pure  extracted  forms  which  have  reverted  to  one  or  other 
of  the  original  parental  types,  the  hybrid  itself  never  being 
permanently  fixed.  New  and  permanent  combinations  may,  of 
course,  sometimes  arise  naturally  through  dihybridism,  but  they 
would  at  first  be  produced  in  very  small  numbers  and  could  only 
be  expected  to  survive  in  the  struggle  for  existence  if  they 
happened  to  possess  some  material  advantage  over  the  parent 
forms.  Thus  it  appears  that  in  a  state  of  nature  the  results  of 
hybridization  tend  to  be  automatically  eliminated. 

Attention  has  frequently  been  called  to  the  alleged  discrepancy 
between  the  results  of  the  Mendelian  experiments  and  those 
obtained  by  the  late  Sir  Francis  Galton  and  others  by  statistical 
methods  based  upon  the  quantitative  estimation  of  characters  in 
a  number  of  successive  generations.  As  a  result  of  his  investiga- 
tions Galton  was  able  to  formulate  the  following  "  Law "  of 
inheritance : — "  The  two  parents  contribute  between  them  on  the 
average  one-half,  or  (0'5)  of  the  total  heritage  of  the  offspring ; 
the  four  grand-parents,  one-quarter,  or  (0'5)2 ;  the  eight  great- 
grandparents,  one-eighth,  or  (Q£fi,  and  so  on.  Thus  the  sum 
of  the  ancestral  contributions  is  expressed  by  the  series  { (0*5)  -f 
(0'5)2  +  (0*5)3,  &c.},  which,  being  equal  to  1,  accounts  for  the 
whole  heritage."1 

This  series  has  been  modified  by  Professor  Karl  Pearson  for 

1  Proceedings  of  the  Royal  Society  of  London,  Vol.  LXI.,  1897.  p.  402. 
B.  P 


210       OUTLINES   OF   E VOLUTION AKY   BIOLOGY 

certain  reasons  into  which  we  cannot  enter  in  this  place,  but  it 
may  be  taken  as  a  substantially  correct  expression  of  the  results 
obtained  by  biometrical  inquiry.  If  we  remember  that  Galton's 
Law  merely  expresses  the  average  results  which  may  be  antici- 
pated from  the  interbreeding  of  a  large  population,  in  which 
hybridization  probably  plays  a  very  small  part,  it  does  not  appear 
in  any  way  P  ntagonistic  to  the  Mendelian  theory.  It  is  of  course 
not  applicable  to  individual  cases  of  hybridization,  where  we  are 
concerned,  not  with  small  variations  in  the  same  characters,  but 
with  the  permutations  and  combinations  of  alternative  character 
units. 

Intimately  bound  up  with  Galton's  Law  of  Inheritance  is 
another  important  generalization  known  as  the  Law  of  Filial 
[Regression,  which  we  owe  to  the  same  distinguished  philosopher. 
The  law  of  ancestral  inheritance  teaches  us  that  on  an  average 
the  individual  derives  half  its  characteristics  from  its  immediate 
parents  and  the  remaining  half  from  its  more  remote  ancestry. 
If  the  immediate  parents,  or  one  of  them,  happen  to  depart  from 
the  average  condition  of  the  race  in  respect  of  any  character 
there  will  be  a  tendency  on  the  part  of  the  offspring  to  inherit 
the  deviation  in  question ;  but  not  to  the  same  extent,  for  the 
influence  of  the  more  remote  ancestry  will  tend  to  counteract 
that  of  the  parents  and  cause  a  partial  return — or  regression — 
towards  mediocrity*  As  a  result  of  his  statistical  investiga- 
tions Galton  estimated  that  the  offspring  of  parents  exhibiting 
a  marked  deviation  from  the  average  would  tend  to  inherit  that 
deviation  to  the  extent  of  only  one-third  of  its  magnitude  in  the 
parents  (the  mean  of  the  two  parental  deviations  being  taken  as 
the  standard  of  comparison).  In  the  words  of  Professor  Karl 
Pearson,  "  It  is  the  heavy  weight  of  this  mediocre  ancestry  which 
causes  the  son  of  an  exceptional  'father  to  regress  towards  the 
general  population  mean ;  it  is  the  balance  of  this  sturdy  common- 
placeness  which  enables  the  son  of  a  degenerate  father  to  escape 
the  whole  burden  of  the  parental  ilL  Among  mankind  we  trust 
largely  for  our  exceptional  men  to  extreme  variations  occurring 
among  the  commonplace,  but  ...  if  we  could  remove  the 
drag  of  the  mediocre  element  in  ancestry,  were  it  only  for  a  few 
generations,  we  should  sensibly  eliminate  regression  or  create  a 
stock  of  exceptional  men." 1 

1  Grammar  of  Science,  1900,  pp.  456—457. 


V 


PAET  IV.— THE   THEORY  AND  EVIDENCE   OF 
ORGANIC  EVOLUTION:  ADAPTATION 

CHAPTER  XV 

Organic    evolution    versus  special  creation — Spontaneous    generation    and 
biogenesis — The  origin  of  living  things. 

AT  the  present  day  we  see  the  surface  of  the  earth  teeming  with 
hosts  of  living  things,  incalculable  in  number  and  of  endless 
diversity  in  form  and  structure.  Every  situation  where  life  is 
possible  is  occupied  by  plants  or  animals  of  some  kind  or  other, 
all  specially  -adapted  in  bodily  organization  to  the  conditions 
under  which  they  have  to  maintain  their  existence.  From  the 
bleak  and  inhospitable  summits  of  high  mountain  ranges  to  ocean 
depths  which  can  be  measured  in  miles ;  from  the  perpetually 
frozen  circumpolar  regions  to  the  torrid  zone  on  either  side  of 
the  equator,  living  things  abound.  Seas,  rivers,  lakes,  dry  land 
and  air  have  all  alike  been  taken  possession  of  by  representatives 
of  the  animal  and  vegetable  kingdoms.  A  single  drop  of  water 
may  contain  thousands  of  organisms-,  and  every  chalk-cliff,  coal- 
seam  or  peat-bog  testifies  to  the  countless  myriads  which  have 
lived  in  past  times  and  whose  remains  have  contributed  in  no 
small  measure  to  the  formation  of  the  earth's  <jrust. 

In  the  animal  kingdom  alone  it  is  probable  that  at  least  a 
million  different  kinds  or  species  are  living  on  the  earth  at  the 
present  day.  Some  half  million  or  so  have  already  been  dis- 
covered, named  and  more  or  less  imperfectly  described,  while 
every  exploring  expedition  brings  back  many  which  have  never 
before  been  seen. 

Nevertheless  there  must  have  been  a  time  when  no  living 
things  whatever  existed  on  the  earth.  According  to  the 
nebular  hypothesis  our  planet  is  still  gradually  cooling  from  a 
molten  state,  preceded  probably  by  an  incandescent  gaseous 
condition,  which  must  have  rendered  the  existence  of  any  proto- 
organism  a  physical  impossibility.  Protoplasm  becomes 

p  2 


il 


212       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

coagulated  and  thereby  destroyed  as  living  substance  at  com- 
paratively low  temperatures,  and  cannot  have'been  present  on  the 
earth  until  the  surface  of  the  latter  had  not  only  solidified  but 
also  cooled  down  to  something  approaching  its  present  degree 
of  heat. 

Living  things  must  therefore  have  first  put  in  an  appearance 
at  some  definite  period  well  advanced  in  the  earth's  history 
According  to  the  biblical  account  given  in  the  Book  of  Genesis 
their  advent  was  due  to  a  series  of  special  acts  of  creation  per- 
formed by  the  Creator  at  the  appropriate  time,  and  this  account 
forms  the  basis  of  the  doctrine  of  Special  Creation  which  still 
survives  amongst  uneducated  people.  Upholders  of  this  doctrine 
maintain  that  each  kind  of  plant  or  animal  was  not  only  created 
as  a  separate  kind  and  in  a  state  of  full  perfection,  but  was 
specially  designed  to  suit  the  conditions  under  which  it  was 
placed,  and  in  this  way  the  marvellous  adaptation  of  plants  and 
animals  to  their  environment  was  supposed  to  be  accounted  for. 
The  fish  was  created  to  swim  in  the  sea,  the  horse  to  run  on 
dry  land,  the  monkey  to  climb  in  the  tree  and  the  bird  to  fly  in 
the  air,  and  each  was  constructed  in  accordance  with  its  foreseen 
requirements.  Moreover,  each  species  was  supposed  to  be  immut- 
able, propagating  its  own  kind  by  reproduction  but  never  changing 
into  a  different  kind.1 

Such  a  view  of  the  origin  of  living  things  could  only  have  arisen 
in  a  state  of  almost  complete  ignorance  of  the  phenomena  which 
have  to  be  accounted  for,  and  at  the  present  day  it  has  been 
entirely  superseded,  as  a  scientific  theory,  by  the  doctrine  of 
Organic  Evolution.  This  doctrine  teaches  us  that  species  or  kinds  / 
are  not  immutable  but  are  subject  to  changes  whereby  one  mayl 
give  rise  to  another,  and  that  all  existing  species  are  the  more  or 
less  modified  descendants  of  pre-existing  ones.  Modern  biologists, 
moreover,  maintain  that  the  further  back  we  trace  the  ancestral 
history  of  living  things  the  less  diversity  of  structure  do  they 
show,  until  finally,  if  we  could  trace  them  back  to  their  origin, 
we  should  find  all  the  different  lines  of  descent  converging  towards 
a  common  starting  point  far  back  in  geological  time.  We  shall 
see  presently  that  the  adaptation  to  environment  which  forms 
such  a  characteristic  feature  of  all  living  organisms  can  be 
explained  just  as  well  in  accordance  with  the  theory  of  organic 

1  Compare  the  quotation  from  Linnaeus  on  p.  222  (1st  footnote)  and  that  from 
Buffon  on  p.  369, 


SPECIAL  CREATION  AND  EVOLUTION 


213 


evolution,  or  transmutation  of  species,  as  in  accordance  with  that 
of  special  creation.  . 

The  fundamental  difference  between  these  two  opposing  and 
irreconcilable  doctrines  can  be  expressed  by  means  of  the 
accompanying  simple  diagrams  (Fig.  82) : — 


15  678123*55'!       23 

/     • 


e 

i   a    3*    s 


B'  C'  D' 

Special      Creation 


2345678  i   234567    i     a    3    4-      I    254    51234     5 


.  82. 


The  letters  A — E  are  supposed  to  represent  five  'closely  related 
existing  species  in  each  case.  According  to  the  doctrine  of 
special  creation  each  of  these  species  has  persisted  without  altera- 
tion from  the  time  when  it  was  first  created,  so  that  the 
ancestral  species  may  be  represented  by  the  letters  A'  B'  C'  D' 
and  E',  and  the  lines  of  descent  of  existing  species  run  parallel 
with  one  another.  The  idea  of  evolution,  on  the  other  hand, 
is  expressed  by  showing  these  lines  of  descent  diverging  from 
a  remote  ancestral  species,  x,  which  may  have  been  quite  different 
in  character  from  any  of  the  existing  species. 

Any  existing  species  at  the  present  day  is  made  up  of  a 
larger  or  smaller  number  of  individual  organisms,  and  we  know 
as  a  matter  of  fact,  proved  by  daily  observation,  that  these 


214        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

individuals  arise  by  some  process  of  reproduction  from-  pre-exist- 
ing parent  individuals.  This  is  represented  in  the  upper  parts 
of  both  diagrams,  where  a  1—8,  b  1—7,  c  1—4,  d  1—5  and 
e  i — 5;  on  each  side,  are  supposed  to  represent  small  groups  of 
individuals  of  the  species  A  B  C  D  and  E  respectively,  each 
group  being  descended  from  some  common  ancestor  (or  pair  of 
ancestors)  which  itself  belonged  to  the  species. 

The  study  of  this  diagram  is  alone  sufficient  to  afford  strong 
presumptive  evidence  in  favour  of  the  view  of  the  evolutionist  as 
against  that  of  the  upholder  of  special  creation,  for  the  evolu- 
tionist in  his  imagination  extends  backwards  into  the  past  the 
processes  which  he  sees  taking  place  constantly  at  the  present 
day,  and  endeavours  to  account  for  the  origin  of  species  in 
accordance  with  what  he  knows  to  be  true  of  the  origin  of 
individuals.  In  other  words,  the  diagram  expressing  the  idea  of 
organic  evolution  is  consistent  throughout,  whereas  that  which 
represents  the  idea  of  special  creation  is  made  up  of  two  incon- 
gruous portions,  and  is  therefore  less  likely  to  be  correct.  The 
.  great  weakness  of  the  doctrine  of  special  creation,  however,  lies 
in  its  failure  to  explain  countless  facts  of  comparative  anatomy, 
embryology,  geographical  distribution  and  palaeontology,  all  of 
•which  are  readily  explicable  in  terms  of  evolution.  We  shall 
deal  with  some  of  these  facts  in  subsequent  chapters. 

It  is  obvious,  then,  that  in  order  to  be  logically  consistent  the 
evolutionist  need  not  postulate  more  than  a  single  starting  point 
for  the  evolution  of  the  whole  organic  world,  though  he  is  not 
obliged  to  limit  himself  in  this  way  should  evidence  be  forth- 
coming that  there  has  been  more  than  one  starting  point.  This 
brings  us  to  the  consideration  of  the  extremely  difficult  and  at 
present  quite  insoluble  problem  of  the  origin  of  the  first  living 
organisms. 

Here  again  we  may  first  inquire  whether  our  experience  of  what 
takes  place  at  the  present  day  throws  any  light  upon  the  problem. 
Many  people  in  the  past  have  held,  ami  some  fe^w  s&ll  toaintain, 
tha't*  living  thin^in*ay,x  even  n'pV,  arise  from  not-living  matter. 
This  is  known  as  the  doctrine  ©f  "  Spontaneous  Generation  "  or 
abiogenesis.  One  of  the  earliest  expressions  of  this  idea  is  met 
with  in  classical  mythology,  in  the  story  of  the  birth  of  Aphrodite 
from  the  sea-foam  ;  and  Vergil,  in  the  Georgics,  accepts  the  prin- 
ciple involved  therein  apparently  in  perfectly  good  faith. 

Vergil's  account  of  the  manner  in  which  a  swarm  of  bees  may 


SPONTANEOUS   GENERATION  215 

be  produced  from  the  decomposing  body  of  a  dead  ox  affords  a 
very  striking  illustration  of  that  want  of  scientific  training  in 
observation  and  reasoning  which  has  led  to  so  many  erroneous 
beliefs.  He  tells  us,  with  much  elaboration  of  detail,  that  if  the 
body  of  an  ox  is  well  beaten  and  enclosed  in  a  suitable  chamber  a 
swarm  of  bees  will  arise  from  it  within  a  certain  number  of  days. 
We  now  know  perfectly  well  what  may  really  happen  under  these 
circumstances.  In  the  first  place  the  supposed  bees  are  not  bees 
at  all,  but  drone-flies,  which  superficially  resemble  bees,  though 
easily  recognizable  as  belonging  to  a  totally  different  order  of 
insects  by  the  fact  that  they  possess  only  two  wings  instead  oi 
four,  and  consequently  any  husbandman  who  followed  Yergil's 
instructions  must  have  been  grievously  disappointed  in  his 
expectations  of  honey.  In  the  second  place  the  drone-flies  are  not 
spontaneously  generated  from  the  body  of  the  ox,  or  from  any- 
thing else,  but  are  hatched  out,  first  in  the  form  of  maggots,  from 
eggs  which  were  laid  by  pre-existing  flies.  These  maggots, 
having  fed  abundantly  on  the  decaying  carcase,  presently  undergo 
their  metamorphosis  and  emerge  as  flies. 

Many  similar  instances  of  alleged  spontaneous  generation,  all 
resting  upon  gross  ignorance  of  the  real  facts  of  the  case,  might 
be  collected  from  the  writings  of  ancient  and  mediaeval  authors. 
The  cruder  stories,  which  could  be  easily  disproved  by  simple 
observation,  were  soon  cast  aside  as  fables  under  the  influence  of 
modern  scientific  methods.  The  invention  of  the  microscope, 
however,  and  the  consequent  revelation  of  a  new  world  of 
hitherto  invisible  organisms,  led  to  a  revival  of  the  doctrine  of 
abiogenesis.  It  was  noticed  that  organic  infusions,  even  after 
boiling,  presently  became  densely  filled  with  various  kinds  of 
micro-organisms,  especially  Bacteria,  and  as  the  boiling  was 
supposed  to  have  killed  any  organisms  that  might  have  been  in 
them  at  first  it  was  argued  that  living  things  arose  in  them 
de  woro,  by  spontaneous  generation.  This  was,  however,  merely 
a  revival  of  Vergil's  story  of  the  swarm  of  bees  in  a  more  refined 
form.  Had  Vergil's  ox  been  protected  from  flies  none  of  the 
alleged  bees  would  have  been  produced,  and  the  careful  experi- 
ments of  suck  observers  as  Tyndall  and  Pasteur  have  conclusively 
demonstrated  that  if  the  organic  infusions  are  adequately  pro- 
tected from  the  countless  microscopical  germs  which  float  in  thB 
air  th$y  will  remain  free  from  living  organisms,  and  consequently 
from  putrefactioTa,-for  an  indefinite  period,  provided  a-lways  that 


216       OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

they  are  themselves  first  properly  sterilized.  This  sterilization, 
however,  cannot  always  be  effected  by  a  single  boiling,  for  the 
germs  of  some  organisms  are  extremely  resistant  to  heat  and 
special  precautions  have  to  be  taken  accordingly. 

Every  story  of  alleged  spontaneous  generation  has,  so  far, 
failed  to  stand  the  test  of  scientific  criticism  and  experiment,  and 
accordingly  biologists  are  now  almost  unanimous  in  maintaining 
the  contrary  doctrine  of  biogenesis,  which  teaches  that,  at  the 
present  day  at  any  rate,  living  organisms  arise  only  by  repro- 
duction from  pre-existing  living  organisms.  Nevertheless  there 
must,  as  we  have  seen,  have  been  a  time  when  there  were  no 
living  things  >0n  the  earth,  and  therefore  living  things  must  have 
either  reached  the  earth  from  some  outside  source  or  haye  arisen 
on  the  earth  itself  from  matter  which  was  previously  not-living 
or 'hi  o^fteV  words, 'by  spontaneous  generation*. x •-, 

,JkhasJ)een  maintained  byvso0ne\tkairorga-nic  life^§^t$mial  and 
is  transferred  from  ffn.e  World  to  another — possibly*  on  meteorites— 
in  the  form  of  minute  germs  fcr  "$^moao£i$'^trt*no  one  has  ever 
seen  such  Cosmozoa,  and  it  is  difficult  to  imagine  any  which 
would  be  capable  of  surviving  such  a  journey.1  It  is  perhaps  less 
difficult  to  believe  that  under  certain  conditions,  of  which  we  at 
present  know  nothing  and  which  perhaps  no  longer  exist  on  the 
earth's  surface,  living  protoplasm  may  have  arisen  from  in- 
organic matter  as  the  result  of  chemical  and  physical  processes. 
In  this  connection  we  must  remember  that  protoplasm — the 
physical  basis  of  life — contains  no  chemical  elements  that  are 
not  also  found  in  inorganic  matter  and  is,  as  a  matter  of  fact, 
constantly  being  built  up  from  inorganic  constituents  in  the 
bodies  of  living  organisms,  though  apparently  only  by  the  action 
of  pre-existing  protoplasm. 

Whether  physical  and  chemical  forces,  such  as  we  are 
acquainted  with,  alone  sufficed  to  bring  about  the  transformation 
of  not-living,  inorganic  material  into  living  proto^pJasm,  or 
whether,  as  some  people  suppose,  some  unknown  "  vital  force  " 
was  (and  still  is)  involved  in  the  process,  must  remain,  for  the 
present  at  any  rate,  an  open  question. 

The  time  has  evidently  not  yet  arrived  for  solving  the  great 
problem  of  the  origin  of  life,  but  we  may,  at  any  rate,  legitimately 
speculate  upon  the  nature  of  the  living  organisms  which  first 

1  Arrhenius,  however,  suggests  that  minute  germs  may  be  transferred  from  planet 
to  planet  by  "radiation  pressure"  ("  Worlds  in  the  Making,"  English  Trans.,  1908). 


THE   FIRST   LIVING   THINGS 


217 


appeared  on  the  earth.  In  the  first  place  we  may  safely  assume 
that  they  were  extremely  simple  in  structure,  for  it  is  generally 
agreed  that  the  evolution  of  the  higher  forms  of  life  has  been 
accompanied  by  a  gradually  increasing  complexity  of  organization. 
It  is  also  certain  that  they  cannot  have  been  animals,  for  animals, 
as  we  have  seen  in  a  previous  chapter,  are  dependent  for 
their  food  supply  upon  other  living  things,  being  themselves 
unable  to  build  up  the  proteid  molecule  from  inorganic  constitu- 
ents. Green  plants  are  the  great  proteid  manufacturers  at  the 
present  day,  but  only  by  virtue  of  the  fact  that  they  contain 
chlorophyll,  which  enables  them  to  utilize  the  energy  of  the  sun's 


r«  w. 


I '4 

9 


r 


FIG.  83.— Different  Forms  of  Disease-producing  Bacteria,   X  about  1500. 
(From  Strasburger,  after  Fischer.) 

a,  pus  cocci ;  6,  erysipelas  cocci ;  c,  gonorrhoea  cocci ;  d,  bacilli  of  splenic  fever;  e,  bacilli 
of  tetanus ;  /,  bacilli  of  diphtheria ;  g,  tubercle  bacilli ;  h,  typhoid  bacilli ;  i,  colon 
bacilli ;  k,  cholera  bacilli. 

rays  in  the  process  of  photosynthesis.  Now  chlorophyll  itself  is  a 
very  complex  substance,  and  we  cannot  suppose  that  the  first  living 
things  already  possessed  it.  If,  then,  they  had  no  other  organisms 
to  feed  upon,  and  if  they*  possessed  no  chlorophyll,  how  did  they 
obtain  the  energy  necessary  to  enable  them  to  maintain  life 
at  all? 

The  simplest  and  at  the  same  time  the  smallest  organisms  known 
to  us  at  the  present  day  are  the  Bacteria  (Fig.  83),  many  of  which 
are  so  minute  as  to  be  hardly  visible  even  under  the  highest 
powers  of  the  microscope.  These  Bacteria  are  neither  plants 
nor  animals,  but  occupy  a  position  lower  than  either.  They  have 
not  even  attained  to  the  dignity  of  perfect  cells,  for  they  exhibit 
no  proper  differentiation  into  cell  body  and  nucleus,  the  chromatin 
substance,  which  in  a  typical  cell  is  aggregated  in  the  nucleus, 


218       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

being  scattered  in  minute  granules  throughout  the  cytoplasm 
(vide  Fig.  27).  They  contain  no  chlorophyll  and  the  colourless 
protoplasm  is  enclosed  in  a  very  thin  and  delicate  cell  wall.  In 
form  they  vary  greatly,  being  sometimes  spherical,  sometimes 
rod-shaped  and  sometimes  corkscrew-shaped,  and  frequently 
they  occur  united  together  in  chains.  Some  of  them  swim  about 
by  means  of  cilia  or  flagella,  others  are  motionless,  and  they 
multiply  by  simple '  fission  and  also  by  the  formation  of  spores, 
which  have  extraordinary  powers  of  resistance  and  thus  serve  to 
secure  the  wide  dispersal  of  these  organisms  by  air  and  water. 

A  great  number  of  different  kinds  of  these  Bacteria  are  already 
known  to  us,  and  doubtless  a  vast  number  still  remain  to  be  dis- 
covered. They  occur  practically  everywhere  ;  earth,  air  and  water 
are  full  of  them,  and  they  exercise  a  most 
profound  influence — sometimes  injurious 
and  sometimes  beneficial — upon  the  lives 
of  other  living  things.  Some  of  them  are 
the  active  agents  in  the  putrefaction  and 
fermentation  which  are  rapidly  set  up  in 
dead  organisms,  and  others  are  responsible 
for  many  of  the  most  grievous  ills  which 
living  flesh  is  heir  to.  Thus  most  of  them 

live   either  as   saprophytes  (upon    dead 
FIG.  84.  —  Nttrosomonas  •    J          v  .f 

europcea,      x     600.    organic   matter)  or    as    parasites  (upon 
(From       Percival's    iiving  organisms)  and  obtain  tl^elr  supply 

ridK  of  energy from  alread? formed  Proteids 

and  other  complex  chemical  compounds. 
Some,  however,  are  said  to  live  free  in  the  soil  and  to  be  able 
to  derive  their  food  supply,  and  with  it  their  energy,  from  purely 
inorganic  substances.  These  are  called  nitrifying_Bacteria.  One 
form,  Nitrosomonas  (Fig.  84),  oxidizes  ammonia  to  nitrous 
acid,  and  another,  Nitrobacter,  oxidizes  nitrous  to  nitric  acid. 
In  this  way  they  perform  an  important  function  in  preparing  the 
food  material  in  the  soil  for  the  use  of  green  plants,  but  their  chief 
interest  for  us  lies  in*  the  fact  that  they  are  able,  in  some  manner 
which  we  do  not  understand,  to  build  up  their  own  proteid  Inola- 
cules  directly  from  inorganic  constituents  without  the  aid  of  chloro- 
phyll. They  show  us  that  such  a  thing  is  at  any  rate  possible, 
and  that  therefore  the  first  livin'g  things  may  ha.ve  been  able  to 
maintain  their  existence  without  either  possessing  chlorophyll 
or  having  othe'rftfganislH^  to,  feed  upon. 


SPONTANEOUS   GENEEATION  219    . 

It  seems  likely  that  of  all  the  organisms  known  to  us  these 
Bacteria  come  nearest  to  the  first  living  things,  and  yet  they 
probably  stand  very  far  from  them  and  represent  a  much  later 
stage  in  the  process  of  evolution.  Minute  and  apparently 
simple  as  the  Bacteria  are,  it  seems  more  than  probable  that  the 
first  living  things  were  very  much  smaller  and  simpler,  so  that 
even  if  we  had  them  under  the  highest  powers  of  our  micro- 
scopes we  should  be  unable  to  recognize  them.  Weismann  has 
suggested  that  they  may  have  been  single  biophors,  i.e.  vital 
units  of  the  first  order,  such  as  he  believes  to  constitute  the 
ultimate  living  particles  of  protoplasm,  and  he  has  actually 
proposed  the  name  "  Biophoridae  "  for  these  hypothetical  free- 
living  primordial  organisms.' 

These  considerations  throw  a  new  light  upon  the  question 
of  spontaneous  generation,  for  if  living  matter  is  first  formed 
in  such  ultra-microscopic  particles  and  can  only  be  recog- 
nized as  living  matter  aftfer  it  has  reached  a  comparatively 
high  stage  of  evolution,  it  is  obvious  that  we  are  not  entitled  to 
say  that  it  is  never  formed  from  notr-living  matter  at  the  present 
day.  We  vcannot  see  it  being  formed,  and  we  probably  never 
shall  see  it  being  formed,  but  it  is  possible  that  it  is  still 
being  " spontaneously-  generated"  all  the  same.  We  are  not 
logically  obliged,  as  we  said  before,  to  content  ourselves  with  a 
single  starting  point  for  organic  evolution,  and  it  would  be  quite 
impossible  to  prove  that  all  the  different  kinds  of  Bacteria,  the 
simplest  organisms  known  to-- us,  have  descended  from  a  single 
ancestor.  They  may  equally  well  have  been  derived  from  a 
number  of  ancestral  protoplasmic  units  which  originated  inde- 
pendently from  inorganic,/not-living  matter.  If  such  an  event 
can  have  taken  place  once  it  may  have  taken  place  many  times, 
and  may  still  be  taking  place  around  us,  though  the  imperfect 
means  of  observation  at  our  disposal  will  not  allow  us  to  demon- 
strate the  fact.  We  may  safely  affirm,  however,  that  no  living  x 
organism  which  we  are  at  present  capable  of  recognizing  as  such 
has  arisen,  or  ever  will  arise,  by  spontaneous  generation,  but  that 
all  organisms  known  to  us  have  been  derived  from  pre-existing 
organisms  by  some  process  of  reproduction  ;  that,  in  the  course 
of  long  ages,  they  have  undergone  slow  changes,  whereby  they 
have  become  more  and  more  diversified  and  usually  more  com- 
plex in  structure,  and  that  in  this  way  the  evolution  of  the 
animal  and  vegetable  kingdoms  has  been~brought  about. 


220       OUTLINES  OF  E VOLUTION ABY  BIOLOGY 

In  concluding  this  chapter  it  may  be  well  to  remind  the  reader 
that  the  idea  of  organic  evolution  is  no  novelty,  but  can  be 
traced  back  to  the  philosophy  of  ancient  Greece.  It  will  be  more 
convenient,  however,  to  postpone  what  we  have  to  say  about  the 
history  of  the  evolution  theory  to  a  later  chapter  and  in  the 
meantime  to  familiarize  ourselves  with  the  theory  itself  and  the 
evidence  upon  which  it  is  based. 


CHAPTER  XVI 

The  continuity   of    life — The    conception    of    species — The    principles    of 
taxonomy — The  taxonomic  evidence  of  organic  evolution. 

IN  accordance  with  the  theory  of  evolution  we  may  picture 
to  ourselves  the  entire  animal  and  vegetable  population  of  the 
earth,  both  past  and  present,  as  forming  one  vast,  tree-like 
organism,  all  parts  of  which,  if  we  knew  enough  about  their 
history,  could  be  traced  into  actual,  although  of  course  not  simul- 
taneous,  protoplasmic  connection  with  all  other  parts.  This 
tree  commenced  its  growth  far  back  in  geological  time,  and  its 
branches  became  ever  more  and  more  ramified  in  succeeding 
ages,  and  more  and  more  diversified  in  character  as  they 
diverged  from  one  another.  Only  the  youngest  twigs  of  the 
tree,  however,  are  still  actually  alive,  being  represented  by  the 
inimal  and  vegetable  population  of  the  earth  for  the  time  being. 
All  the  older  parts  have  died  away,  most  of  them  without  leaving 
any  traces  of  their  former  existence,  and  thus  has  arisen  the 
actual  discontinuity  which  is  found  to  occur  between  the  sur- 
viving groups  of  organisms  at  any  given  period. 

The  tendency  to  structural  variation  which  all  organisms 
diibit,  whatever  may  be  its  cause,  is  responsible  for  the 
:ogressive  diversity  which  is  gradually  set  up  between  the 
ifferent  branches  of  the  organic  tree.  The  combined  action  of 

e  forces  of  heredity  and  variation  bring  about  "  descent  with 

edification."     How  it  is  that  such  modification  always  leads, 

the  long  run,   to   a,  more   or   less  perfect  and  often  very 

jecialized  adaptation  of  both  plants  and  animals  to  the  con- 

.itions  under  which  they  live  will  be  discussed  subsequently. 

At  the  moment  we  have  to  deal  with  things  as  we  actually  find 

them. 

In  any  scheme  of  zoological  or  botanical  classification  the 
lowest  unit  must,  of  course,  be  the  individual.  The  untrained 
observer,  acquainted  only  with  the  more  familiar  plants  and 
animals,  sees  no  difficulty  in  arranging  these  in  apparently 


222        OUTLINES   OF   ETOLUTIONAEY  BIOLOGY 

sharply  circumscribed  groups.  A  child  collecting  flowers  by 
the  wayside  will  tell  you  without  difficulty  how  many  kinds  he 
has  found,  and  these  kinds — speaking  generally — correspond 
to  the  different  species  recognized  by  the  botanist. 

Even  amongst  the  higher  and  better  known  plants  and 
animals,  however,  the  so-called  species  are  not  always  sharply 
distinguishable  from  one  another,  for  we  not  infrequently  meet 
with  intermediate  forms,  or  connecting  links,  between  what  are 
usually  recognized  as  distinct  species.  Linnaeus  himself,  who 
pronounced  most  explicitly  in  favour  of  the  doctrine  olMspecial 
creation,1  was  forced  by  his  profound  knowledge  of  the  animal 
and  vegetable  kingdoms  to  recognize  this  fact  in  the  case  of 
certain  groups  of  plants 2 ;  and  when  we  come  to  study  some  of 
the  lower  members  of  the  animal  kingdom^ as,  for  example,  the 
sponges,  the  difficulty  of  distinguishing  species  sharply  from  one 
another  frequently  becomes  so  great  that  the  definition  of  any 
particular  one  is  little  more  than  a  matter  of  individual 
opinion.  The  more  we  study  the  animal  and  vegetable  king- 
doms, in  short,  the  more  clearly  is  the  fact  impressed  upon  us  that 
if  we  could  have  before  us  all  past  and  present  individuals  we 
should  find  it  impossible,  except  in  an  arbitrary,  manner,  to 
arrange  them  in  species  at  all,  for  each  kind  would  be  found 
to  be  connected  with  others  by  a  series  of  small  gradations. 

According  to  Darwin,  the  differences  which  separate  existing 
species  from  one  another  have,  at  any  rate  in  most  cases,  arisen 
by  the  gradual  accumulation  of  small  successive  variations.  If 
this  be  the  case  the  fact  that  we  are  able  logically  to  distinguish 
such  species  from  one  another  at  all  can  only  be  due  to  that  dis- 
appearance of  the  older  portions  of  the  organic  tree  whereby 
/  discontinuity  has  arisen  between  the , surviving  branches.  In 
any  case  it  is  quite  certain  that  such  destruction  of'former 
connecting  links  has  played  a  very  large  part  in  the  separation  of 
those  groups  of  individuals  to  which  the  term  species  is  usually 
applied.  This  process  is  clearly  illustrated  in  the  accompanying 
diagram  (Fig.  85),  in  which  each  enclosed  area  is  supposed 

1  "  Species  tot  sunt,  quot  diversas  formas  ab  initio  produxit  Infinitum  Ens  ;  quce 
deinde  formse  secundum  generationis  inditas  leges  produxere  plures,  at  sibi  semper 
similes,  ut  Species  nunc  nobis  non  sint  plures,  quam  quse  fuere  ab  initio." 

Linnaeus.     Gen.  Plant.  (1737). 

2  "  Species  Rosarum  difficillime  limitibus  circumscribuntur  et  forte  natura  vis 
eos  posuit."    Linnaeus.     Spec.  Plant.,  Ed.  2,  p.  705  (1762).     I  am  indebted  to  my 
friend  and  colleague  at  the  Linnean  Society,  Dr.  B.  Daydon  Jackson,  for  showing 
me  these  quotations. 


DEFINITION   OF    SPECIES 


223 


to  represent  the  limits  of  an  existing  species.  Within  each  one 
the  individuals  differ  but  slightly  from  one  another,  not  more, 
perhaps,  than  might  be  expected  amongst  the  offspring  of  the 
same  parents.  The  three  species  represented  have,  however, 
become  more  or  less  widely  separated  from  one  another  by  the 
dying  away  of  the  branches  from  which  they  arose,  and  which 
are  represented  in  the  diagram  by  dotted  lines. 

Darwin  has  told  us  that  he  looked  "  at  the  term  species  as 
one  arbitrarily  given,  for  the  sake  of  convenience,  to  a  set  of  , 
individuals  closely  resembling  each  other,  and  that  it  does  not/ 
essentially  differ  from  the> 
term    variety,    which    is 
given  to  less  distinct  and 
more    fluctuating    forms. 
The  term  variety,  again, 
in  comparison  with  mere 
individual    differences,    is 
also  applied  arbitrarily,  for 


tion  of  Species  by  the   Dying  out  of 
connecting  Links. 


convenience'  sake." 

This,  of  course,  is  no  defi- 
nition of  the  term  species 
and  was  not  intended  as 
such.  A  definition  in  ac- 
cordance with  the  above 
views  might,  however,  be 
given  as  follows  : — *\  A 
species  is  a  group  of  FIG.  85. — Diagram  to  illustrate  the  Separa- 

*^  •-         -*__     .  A.  "  * ~J*      d ~  ~" !-.«.     Xl*  *      T^-,*^,^.      .rti-i4-      ^-C 

individuals  that  closely 
resemble  one  another 
owing  to  their  descent .  from  common  ancestors,  which  has 
become  more  pr  less  sharply  separated  from  all  other  co-existing 
species  by  the  disappearance  of  intermediate  forms.'t  Such  a 
definition  would  be  in  accord  with  the  practice  of  many  systematic 
naturalists,  who  are  in  the  habit  of  uniting  species  previously 
considered  as  distinct  whenever  intermediate  forms  are  found, 
the  former  species  being  reduced  to  the  rank  of  varieties  of  one 
and  the  same  species. 

It  is  obvious  that  the  element  of  time  must  be  taken  into 
consideration  yi  forming  such  a  conception  of  species.     At  any 

i  "  Origin  of  jjracies,"  Ed.  6,  p.  42.     The  reader  should  compare  the  very  similar 
Tiews  of  Lamajlfc  on  the  species  question,  given  in  Chapter  XXIV 


- 

i\\l 


224   OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

given  time  the  species  living  on  the  earth  would  form  naturally 
limited  groups,  which  we  should  be  able  to  define  more  or  less 
sharply  if  we  had  sufficient  knowledge  of  the  then  existing 
fauna  and  flora  to  enable  us  to  trace  their  boundaries.  If, 
however,  we  were  to  include  extinct  forms  in  our  survey,  it  is 
obvious  that  the  number  of  existing  species  which  we  should 
recognize  would  be  inversely  proportional  to  the  extent  of  our 
palaeontological  information,  for  the  gaps  between  the  survivors 
would  gradually  be  filled  up  by  the  discovery  of  intermediate 
fossil  forms. 

Naturalists  have  long  recognized  the  fact  that  there  is  another 
way  in  which  more  or  less  sharply  defined  groups  of  individuals 
may  arise,  and  that  is  by  the  occasional  and  suthJeiFappSarance  of 
"  sports,"  or  "  mutations  "  as  they  are  now  generally  called,1  which 
"  breed  true,"  handing  on  their  special  peculiarities  from  genera- 
tion to  generation.  Darwin  was  of  opinion',  that  such  sports  differ 
only  inxdegree  fcpm  the  ordinary  variations  t<j  the  accumulation  of 
^^which  hev  attributed  thechie'f  importance  in  the  Evolution  of  species. 
At  the  present  day,  as  we  have  already  had  occasion  to  point  out, 
some  observers,  and  especially  Professor  Hugo  de  Vries,2  consider 
them  to  be  totally  different  in  kind  from  fluctuating  or  small 
successive  variations,  and  regand  them  as  affording  the  sole  means 
by  which  new  species  originate.  ; 

According  to  the  mutation  theory  any  species  may,  from  time 
to  time,  throw  off  such  sports,  either  singly  or  in  groups,  and  they 
are  from  the  first  distinguishable  by  well  defined,  though  it  may 
be  minute,  characters  from  the  parent  form.  These  mutations 
are  regarded  by  de  Vries  as  constituting  "  elementary  species," 
which  are  already  fully  characterized  and  will  not  change  again 
unless  by  giving  rise  to  fresh  mutations.  De  Vries  maintains 
that  many  of  the  species  of  flowering  plants  as  defined  by 
Linnaeus  are  really  "  aggregate  species,"  each  made  up  of  a  larger 
or  smaller  number  of  such  elementary  species.  The  common 
weed  known  as  Draba  verna,  for  example,  regarded  by  Linnaeus 
as  constituting  a  single  species,  occurs  under  about  200  more  or 
less  distinct  forms.  All  of  these  so-called  elementary  species  are 
believed  to  come  true  from  seed  and  to  have  arisen  as  sudden 
mutations. 

1  Compare  Chapter  XI. 

2  Vide  "  The  Mutation  Theory,"  by  Hugo  de  Vries.      English   translation  by 
Farmer  and  Darbishire  (Kegan  Paul,  Trench,  Triibner  &  Co.  Ld.,  London,  1910). 


CLASSIFICATION  225 

According  to  this  view  a  newly  arisen  species,  at  its  first  origin, 
is  already  sharply  distinguished  from  the  parent  species  and  the 
extinction  of  intermediate  forms  is  not  required  to  separate  the  two. 

The  elementary  species,  however,  though  apparently  constant 
in  their  characters,  are  often  so  similar  to  one  another,  and 
they  are  moreover  so  numerous,  that  for  practical  purposes  of 
classification  it  would  be  undesirable  to  regard  them  as  distinct 
species  in  the  ordinary  sense  of  the  word.  It  is  far  more  con- 
venient to  maintain  the  older  view  that  they  are  merely  varieties 
or  subspecies,  and  their  occurrenceTin  no  way  invalidates  our 
conception  of  the  slow,  tree-like  evolution  of  the  organic  world. 
If  de  Vries  be  right  as  regards  the  importance  he  attributes  to 
them  as  the  sole  means  by  which  species  have  originated,  which 
is  extremely  doubtful,  it  merely  means  that  evolution  has  taken 
place  in  a  rather  more  jerky  fashion  than  we  previously  supposed. 
It  will  be  necessary  to  return  to  this  question  when  we  come  to 
discuss  the  factors  of  organic  evolution.1 

It  is  not  sufficient  for  the  purposes  of  classification  to  arrange  the 
individual  plants  or  animals  with  which  we  are  acquainted  in  their 
respective  species.  These  species  in  turn  must  be  arranged  in 
groups  of  higher  Border,  which  are  termed  genera.  Each  genus 
stands  to  the  species  included  within  it  in  much  the  same  relation 
that  the  species  themselves  stand  in  with  regard  to  individuals, 
and  related  genera,  though  often  sharply  definable,  are  only  so  by 
virtue  of  the  disappearance  of  connecting  links.  The  characters 
by  which  genera  can  be  distinguished  from  one  another  are  of 
a  more  deep-seated  and  fundamental  nature  than  those  which 
distinguish  species.  Individual  opinion,  however,  differs  greatly 
as  to  the  exact  limits  which  should  be  assigned  to  each,  and  it  is  a 
common  thing  for  an  old  established  genus  to  be  subdivided  into 
several  as  the  result  of  the  discovery  of  new  species  and  the  more 
exhaustive  study  of  old  ones.2  It  is  all  a  matter  of  convenience, 
and  one  may  justifiably  make  a  separate  genus  for  any  group 
of  closely  related  species  which  can  be  distinguished  by  some 
common  character  from  all  other  co-existing  groups  of  species. 
Many  naturalists,  however,  consider  that  the  mutual  relation- 
ships of  species  are  more  accurately  expressed  by  making  use  of 
subgenera  as  intermediate  groups  between  genera  and  species. 

1   Vide  Chapter  XXVII. 

8  A  striking  illustration  of  this  is  afforded  by  the  recent  history  of  the  old  genera 
Peripatus  and  Amphioxus,  each  of  which  is  now  subdivided  into  several. 

B.  0 


226        OUTLINES  OF   EVOLUTIONARY  BIOLOGY 

Genera  in  turn  are  grouped  in  families,  separated  from  one 
another  by  still  more  fundamental  characters,  and  with  sub- 
families as  intermediate  groups  where  necessary.  Families  are 
grouped  in  suborders  and  orders,  orders  in  subclasses  and  classes, 
classes  in  phyla  (or  subkingdoms),  and  phyla  in  kingdoms.  Of 
kingdoms  only  two  are  recognized,  animals  and  plants,  and  even 
these  cannot  be  sharply  separated  from  one  another,  because 
amongst  the  lowest  forms  of  life  (Protista)  the  distinction  between 
animals  and  plants,  as  we  have  already  seen,  ceases  to  exist. 

The  arrangement  of  individual  organisms  in  species,  and  the 
grouping  of  these  to  form  genera,  families,  orders,  classes,  phyla 
and  kingdoms,  constitutes  the  work  of  the  systematist  and  is 
usually  spoken  of  as  classification,  while  the  study  of  the 
principles  in  accordance  with  which  classification  should  be 
carried  out  forms  a  special  branch  of  biological  s«iBnce  sometimes 
known  as  taxonomy. 

The  first  and  greatest  of  the  modern  systematist^  was  Karl  von 
Linne,  now  usually  known  as  Linnaeus,  a  Swedish  naturalist  who 
was  born  in  1707  and  died  in  1778.  In  his  great  work,  the 
"  Systema  Naturae,"  which  is  now  recognized  as  the  starting  point 
of  modern  systematic  zoology  and  botany,  he  described  all  the 
species  of  plants  and  animals  then  known,  and  endeavoured  to 
arrange  them  in  a  "  natural  system"  in  accordance  with  their 
mutual  resemblances  and  differences.  As  anv  aid  to  the 
accomplishment  of  this  formidable  task  he  invented  the  binomial 
system  of  nomenclature,  in  accordance  with  which  eaclj  kind  of 
plant  or  animal  is  referred  to  by  the  name  of  the  genus  as  well  as 
by  that  of  the  species  to  which  it  belongs.  The  necessity  for  the 
constant  repetition  of  longer  or  shorter  descriptions-'  as  a  means 
of  identification  every  time  there  is  occasion  to  refer  to  a 
particular  species  is  thus  avoided,  while  at  the  same  time  the  use 
of  the  generic  name  makes  it  possible  to  employ  the  same 
specific  name  in  a  number  of  different  genera  without  risk  of 
confusion,  a  very  important  point  in  consideration  of  the 
enormous  number  of  species  which  have  to  be  distinguished  from 
one  another. 

The  binomial  system  of  nomenclature  has  proved  so  well 
adapted  to  its  purpose  that  it  has  survived  practically  unchanged 
to  the  present  day,  although  the  number  of  known  species  has 
increased  enormously  in  the  interval  and  the  Linnaean  system 
of  classification  has  undergone  profound  modification  at  the  hands 


NOMENCLATIVE  227 

of  later  investigators.  Very  many  of  the  more  familiar  plants 
and  animals  are,  however,  still  known  by  the  names  which 
Linnaeus  gave  to  them. 

It  still  frequently  happens  that  newly  discovered  species  hape 
to  be  named  and  new  genera  defined,  and  various  attempts  have 
been  made  to  formulate  a  code  of  rules  for  the  guidance  of  those 
engaged  in  this  work.  •  This  is  necessary  in  order  to  secure,  so 
far  as  possible,  uniformity  of  method,  and  to  avoid  confusion 
such  as  would  arise  by  giving  the  same  name  to  different  genera 
or  different  species  within  a  genus,  or  different  names  to  the  same 
genus  or  species.  The  chief  points  so  far  agreed  upon  are  that 
generic  and  specific  names  (as  well  as  those  of  larger  groups) 
should  be  given  in  either  Latin  or  Greek  and  that  strict  attention 
should  be  paid  to  the  law  of  priority,  the  name  first  given 
taking  precedence  over  any  which  may  be  proposed  subsequently, 
as  is  often  the  case  owing  to  ignorance  of  the  work  of  previous 
writers. 

The  greatest  latitude  has,  however,  been  allowed  in  the  past 
with  regard  to  the  coining  of  generic  and  specific  names.  Even 
the  resources  of  the  Latin  and  Greek  languages  have  proved  hardly 
sufficient  to  meet  all  requirements,  and  many  generic  names  have 
been  given  which,  though  euphonious  enough,  have  no  meaning 
whatever.  It  is  said  that  one  eminent  naturalist  (Dr.  Gray) 
used  to  make  up  such  names  by  drawing  letters  out  of  a  hat,  and 
it  is  certain  that  Dr.  Leach  derived  the  generic  names  of  a  whole 
series  of  parasitic  Crustacea  (Cirolana,  Conilera,  Nerocila,  &c.)  from 
anagrams  on  his  wife's  name,  Caroline.  The  most  satisfactory 
names,  on  the  other  hand,  are  those  which  convey  some  definite 
information  as  to  the  characters  of  the  genus  or  species  concerned 
or  as  to  the  place  where  it  is  found. 

If  we  have  to  classify  a  number  of  inorganic  bodies  we  may 
set  about  our  work  in  a  variety  of  ways.  We  may  arrange  them 
according  to  colour,  or  size,  or  weight,  or  shape,  or  texture,  or 
chemical  composition,  or  according  to  the  places  they  come  from, 
the  precise  method  adopted  depending  upon  what  it  is  that  we 
wish  to  express  by  means  of  our  classification. 

It  is  exactly  the  same  with  plants  and  animals,  the  way  in 
which  we  classify  them  will  depend  upon  our  point  of  view.  A 
fisherman  classifies  fishes  largely  according  to  their  food-value,  and 
various  other  animals  according  to  whether  or  not  they  are  good  for 
bait,  and  such  classification  serves  its  purpose  quite  satisfactorily. 

Q  2 


228        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


Individuals 
Species 


Genera 


Families 


Orders 


Individual 
Brown  Bears 


Species  Arctos 

) 

Genus  Ursus 

Family  Ursidae 


Order  Carnivora 


Class  Mammalia 


'urn   Vertebrata 


The  aim  which  the  zoologist  or  botanist  sets  before  himself  in 
classification  is  the  expression  of  the  "  natural  affinities  "  of  the 
plants  or  animals  investigated.  The  existence  of  such  natural 
affinities  was  clearly  recognized  long  before  the  explanation  of 
them  was  known,  and  even  by  upholders  of  the  doctrine  of  special 

creation.  Thus  it  was  , 
well  known  to  Linnaeus 
that  individuals  fall 
naturally  into  species, 
that  species  may  be 
grouped  in  genera, 
genera  in  families,  and 
so  on  in  ever  widening 
circles.  It  also  soon 
came  to  be  recognized 
that  natural  Affinities 
could  be  best  determined 
by  taking  into  account 
as  many  characters  as 
possible  instead  of  rely- 
ing merely  on  one  or 
two.  A  system  of  classi- 
fication based  on  a  small 
number  of  characters 
only  is  always  more 
or  less  artificial,  and 
though  it  may  serve  its 
purpose  for  a  time  it 
will  have  to  be  amended 
by  future  workers  who*' 
are  able  to  bring  a  more 
complete  knowledge  to 

bear  upon  the  problem. 
FIG.  86.— Diagram  to  show  the  Tree-like  Form   T,  -f     nnrnaa    Oumif 

assumed  by  a  natural  Classification.  inus     li     COmeS    about 

that  our  views  on  the 

classification  of  the  animal  and  vegetable  kingdoms  are  always 
undergoing  change,  old  systems  are  constantly  being  discarded 
as  too  artificial  and  more  natural  ones  proposed  in  their  stead. 

As  our  knowledge  progresses  it  becomes  more  and  more  evident 
that  a  natural  classification  assumes  a  tree-like  form.  The 
division  of  the  whole  organic  world  (with  the  exception  of  the 


Animalia 


THE    TAXONOMIC   TEEE  229 

lower  Protista)  into  plants  and  animals  is  represented  by  the  first 
branching  of  the  tree,  the  division  of  each  of  these  great  king- 
doms into  phyla  by  the  next  branching,  and  the  subdivision  of 
phyla  into  classes,  orders,  families,  genera,  species  and  individuals 
by  the  further  ramifications,  as  shown  in  the  accompanying 
diagram  (Fig.  86),  which  represents  a  mere  fragment  of  the  whole 
tree  selected  to  show  the  systematic  position  of  a  single  species  of 
animal,  the  common  brown  bear,  Ursus  arctos. 

This  great  truth  affords  one  of  the  most  striking  pieces  o 
evidence  in  favour  of  the  theory  of  organic  evolution,  for  the 
tree-like  form  assumed  by  a  natural  classification  bears  an 
unmistakable  resemblance  to  the  tree-like  development  of 
the  whole  organic  world  which  evolutionists  believe  to  have 
taken  place.  The  two  results  represent,  indeed,  but  slightly 
different  aspects  of  the  same  truth;  the  resemblance  betwee 
them  is  no  mere  coincidence,  but  the  fact  that  we  are  able 
to  classify  organisms  in  a  tree-like  manner  indicates  very  clearly 
that  these  organisms  have  been  produced  by  tree-like  evolution. 

The  systematist  is  now  able  to  replace  the  vague  groping  after 
"  natural  affinities  "  by  a  much  clearer  conception  of  the  aims  of 
taxonomy.  What  he  has  to  strive  after  is  the  elucidation  of  the 
actual  pedigrees  of  existing  organisms,  the  unravelling  of  what  is 
termed  their  phylogeny  or  ancestral  history.  This  means  neither 
more  nor  less  than  the  ultimate  reconstruction  of  the  whole  vast 
tree  of  life.  Piece  by  piece  this  is  gradually  being  done,  and  it  is 
the  great  German  biologist,  Ernst  Haeckel,  who  has  led  the  way 
in  this  particular  department  of  biological  investigation.  Some 
of  the  main  branches  of  the  tree  have  already  been  reconstructed 
in  a  satisfactory  manner,  but  it  is  a  work  of  immense  difficulty 
and  practically  unlimited  extent,  and  at  every  step  opinions 
differ  as  to  the  value  of  conflicting  evidence  and  the  interpreta- 
tion of  obscure  facts.  The  greater  part  of  the  tree  is  dead  and 
gone  beyond  recall,  and  such  fragmentary  information  concerning 
extinct  forms  as  may  be  supplied  by  the  discovery  of  fossils  can 
only  be  supplemented  by  indirect  evidence  derived  from  the  study 
of  existing  organisms. 

We  must  now  inquire  a  little  more  fully  into  the  relation 
between  the  tree-like  form  assumed  by  a  natural  system  of 
classification,  or  the  taxonomic  tree,  as  we  may  term  it,  and  the 
phylogenetic  tree,  which  represents  the  actual  pedigrees  of 
organisms.  In  the  former  (Fig.  86)  the  main  trunk  of  the  tree, 


230       OUTLINES   OF   E VOLUTION AKY  BIOLOGY 


as  we  have  seen,  represents  the  entire  organic  world ,'•  in  the 
latter  it  represents  the  remote  Protistan  ancestors  from  which 
both  plants"  and  animals  have  sprung. N}  The  first  great  branching 
represents  in  the  one  case  the  systematist's  main  division  of 
organisms  into  vegetable  and  animal  kingdoms,£and  in  the  other 
the  gradual  differentiation  of  the  ancestral  Protista  (which  at  one 

time  constituted  the 

Existing    Groups  ,  .  .  ,  ,N 

Coetenter^  Worm,  '  Verges      CntlTO  Organic  WOrld) 

into  the  primitive 
unicellular  plants  or 
Protophyta  on  the 
one  hand  and  the 
primitive  unicellular 
animals  or  Protozoa 
on  the  other ;  and  so 
on  with  subsequent 
ramifications. 

The  parallelism 
/between  the  two  is 
sufficiently  striking 
to  justify  us  in  believ- 
ing that  it  would  be 
complete  if  only  our 
knowledge  of  classi- 
fication and  phylo- 
geny  were  so ;  we 
should  then  doubtless 
see  at  once  that  the 
taxonomic  tree  and 
the  phylogenetic  tree 
are,  after  all,  one  and 
the  same  thing,  for  we  should  arrange  all  organisms  strictly  in 
accordance  with  the  course  of  their  evolution. 

One  point  remains  to  be  noticed  in  connection  with  the  phylo- 
genetic tree.  The  branching  has  been  monopodial  rather  than 
dichotomous  or  polychotomous,  each  branch,  in  addition  to  giving 
off  lateral  branches,  being  itself  continued  on,  so  to  speak,  at 
each  forking.  The  descendants  of  any  given  ancestral  group 
did  not  all  undergo  modification  to  the  same  extent,  but  some 
adhered  more  or  less  closely  to  the  ancestral  condition  and  have 
continued  to  do  so  to  the  present  day.  Even  the  primitive 


AacestraJ.'Cceffiinratei 


Ancestrj/  Protista 

FIG.  87. — JJiagram  illustrating  the  Relation 
between  Classification  and  Phylogeny. 


CLASSIFICATION   AND   PHYLOGENY 

ancestral  Protista  are  still  represented  by  descendants  ^ 
probably  differ  but  little  from  their  remote  progenitors  and  which 
have  not  yet  reached  the  level  of  either  plants  or  animals,  and 
there  are  still  living  unicellular  forms  in  abundance  which,  though 
they  may  perhaps  be  distinguished  as  plants  or  animals,  have 
never  been  able  to  learn  the  secret  of  forming  multicellular 
bodies.  So  it  is  with  most  of  the  great  ancestral  groups ;  they 
are  represented  still  by  descendants  which  have  undergone  com- 
paratively little  change  in  structure  since  remote  geological 
periods.  The  algae  are  still  algae,  the  coelenterates  are  still 
coelenterates  and  the  fishes  are  still  fishes,  though  each  of  these 
great  groups  has  in  past  time  given  rise  to  descendants  which 
have  gradually  become  modified  into  higher  types. 

Many  subordinate  groups,  such  as  the  trilobites  and  ammonites 
and  the  winged  reptiles  of  the  secondary  period,  have  of  course 
become  extinct  during  past  geological  ages,  but  the  fact  remains 
that  even  at  the  present  day  there  still  exist  on  the  earth  organic 
types  which  represent,  in  a  more  or  less  unmodified  form,  all  the 
principal  stages  of  organic  evolution,  and  thus  it  is  that,  even  if 
we  had  only  living  plants  and  animals  to  consider,  our  classifica- 
tion of  the  organic  world  would  still  assume  a  tree-like  form, 
with  the  simplest  organisms  at  the  bottom  and  the  most  complex 
at  the  top  of  the  tree.  It  would  be  difficult  indeed  to  explain 
this  fact  in  accordance  with  the  theory  of  special  creation  and 
immutability  of  species. 

The  monopodial  branching  of  the  organic  tree  and  the  relation 
which  the  natural  classification  of  existing  animals  bears  to  the 
phylogenetic  or  ancestral  series  is  diagrammatically  represented 
in  Fig.  87.  On  the  right  hand  side  are  shown  some  of  the 
principal  stages  in  the  evolution  of  vertebrates  from  ancestral 
Protista,  and  at  the  top  of  the  tree  are  shown  the  existing  groups 
by  which  these  stages  are  actually  represented  at  the  present 
day.  Only  a  few  stages  are  represented  and  the  great  majority 
of  the  lateral  branches  of  the  tree  are  supposed  to  have  been 
lopped  off. 


CHAPTEE  XVII 

Connecting    links  —  Homolggy    and    analogy  —  Convergent    evolution  — 
Change  of  function — Vestigial  structures— Reversion. 

IN  presenting  the  evidence  of  organic  evolution  it  is  convenient 
to  separate  that  derived  from  the  study  of  comparative  anatomy 
from  that  afforded  by  taxonomy,  but  the  separation  is  not 
a  logical  one,  for  classification  is  necessarily  based  upon  com- 
parative anatomy,  and  much  of  the  evidence  might  be  equally  well 
dealt  with  under  either  heading. 

The  mere  fact  that  we  are  able  to  arrange  existing  organisms 
in  progressive  series,  of  which  the  extremes  are  connected  by 
intermediate  forms,  in  itself  suggests  that  one  form  has  been 
derived  from  another.  This  gradation  is  equally  obvious  whether 
we  study  entire  organisms  or  confine  our  attention  to  their  com- 
ponent parts  or  organs.  It  is  illustrated  very  clearly  by  the 

of  the  modifications  exhibited  by  the 
rtain  sponges  (Fig.  88).  These  micro- 
nge,  each  of  which  arises  within  a  single 
ling  diversity  and  beauty  of  form.  The 
fferent  from  one  another  as  we  can  well 

imagine,  but  in  the  particular  group  of  sponges  from  which  our 
illustration  is  taken  (the  Tetraxonida)  all  may  be  derived  from  the 
game  four-rayed  architype  (number  1  in  the  centre  of  the 
figure)  and  numerous  intermediate  forms  mark  out  the  lines 
along  which  evolution  has  taken  place.  An  analogous  diagram, 
with  a  triaxonid  architype,  could  easily  be  constructed  for  those 
remarkable  deep-sea  sponges  known  as  the  Triaxonida  or 
Hexactinellida,  and  another  for  the  calcareous  sponges. 

Amongst  the  higher  animals  also  innumerable  illustrations  of 
the  same  general  principle  of  gradation  in  structure,  even  in  co- 
existing types,  are  met  with,  and  are  to  be  explained  in  the 
way  indicated  in  the  last  chapter,  as  expressions  of  phylogenetic 
relationship.  Take,  for  example,  those  now  widely  contrasted 
groups  of  vertebrate  animals  the  Reptilia  and  the  Mammalia.  The 


CONNECTING   LINKS 


233 


reptiles  as  a  group  are  distinguished  from  the  mammals  in 
many  ways.  Amongst  other  things  they  usually  have  a  scaly 
epidermic  covering,  but  they  never  have  hair ;  they  lay  large, 


n      o 


J9. 


FIG.  88. — Series  of  tetraxon  Sponge  Spicules,  showing  how  all  may  be 
derived  from  the  same  fundamental  type,  highly  magnified.  (From 
Dendy,  Article  "  Sponges  "  in  Encyclopaedia  Britannica,  Ed.  XL) 

heavily -yolked  eggs  and  do  not  suckle  their  young ;  they 
have  a  cloaca  into  which  the  urinary  and  genital  ducts, 
as  well  as  the  alimentary  canal,  discharge;  they  have  a  very 
complex  shoulder  girdle  (Fig.  89),  consisting  of  a  consider- 
able number  of  more  or  less  separate  bones  or  cartilages 


234        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


a 


'Ep.C. 


including   suprascapula,   scapula,  coracoid,  epicoracoid,  clavicle 
and  interclavicle. 

The  mammals,  on  the  other  hand,  are  characterized  by  the 
possession  of  an  epidermic  covering  of  hairs ;  they  generally 
have  very  minute  eggs  almost  destitute  of  yolk,  and  they  always 
suckle  their  young ;  they  usually  have  no  cloaca  but  separate 
openings  for  the  alimentary  canal  and  urino-genital  ducts,  and 
the  shoulder  girdle  (Fig.  90)  is  very  greatly  reduced,  being  often 
represented  by  only  a  single  bone,  the  scapula,  with  a  small 

remnant  or  vestige  of 
the  coracoid  completely 
fused  with  it,  though  a 
slender  clavicle  is  some- 
times present  as  well. 

There  exists  in  Aus- 
tralia and  some  of  the 
adjacent  islands,  how- 
ever, a  small  but  very 
interesting  group  of 
animals  known  as  the 
Monotremata,  repre- 
sented by  the  duck-billed 
Platypus  (Ornithorhyn 
chus,  Fig.  91)  and  the 

spiny  anteater  (Echidna, 
FIG.  89.— Shoulder  Girdle  and  Sternum  of  a 

Lizard  ( Varanus)  seen  from  below. 


fnt 


Cl.  Clavicle;  Cor.  Coracoid;  Ep.  C.  Epicoracoid; 
Gl.  Glenoid  cavity ;  Int.  Interclavicle ;  E.  Ribs ; 
Sc.  Scapula;  St.  Sternum. 


Fig.  92).  The  fact  that 
these  animals  possess 
hair  and  suckle  their 
young  justifies  us  in 
classifying  them  amongst  the  Mammalia,  but  at  the  same  time 
they  exhibit  certain  characters  in  respect  of  which  they  differ 
from  all  typical  mammals  and  agree  with  the  reptiles.  Thus  they 
lay  large,  heavily-yolked  eggs,  'the  alimentary  canal  and  urino- 
genital  ducts  discharge  into  a  common  cloaca,  and  the  shoulder 
girdle  (Fig.  93)  is  made  up  of  a  considerable  number  of  separate 
bones  almost  exactly  as  in  reptiles,  most  remarkable  amongst 
which  is  the  interclavicle,  which  is  found  in  no  other  mammals. 

Here  then  we  have  animals  still  existing  which  undoubtedly 
occupy  an  intermediate  position  between  the  reptiles  and  the 
typical  mammals,  and  the  three  groups  exhibit  a  progressive  series 
of  structural  peculiarities.  Such  definite  connecting  links  as  the 


LIMBS   OF   VERTEBRATES 


235 


Monotremata,  however,  are  more  usually  met  with  amongst  the 
extinct  animals  of  past  geological  periods.  They  indicate  the 
paths  along  which  the  more  highly  organized  groups  have  pro- 
gressed during  their  evolution  from  more  lowly  organized 
ancestral  groups,  which  latter  may  or  may  not  still  be  represented 
by  surviving  forms  at  the  present  day. 

It  would  be  difficult  to  explain  the  occurrence  of  such  graduated 
series  of  organs  and  organisms  by  the  theory  of  special  creation, 
and  it  would  be  no  less  difficult  to  explain  in  this  way  those 
remarkable  facts  of  comparative  anatomy  which  are  grouped 


FIG.  90. — Shoulder  Girdle  and  part  of  Sternum  of  a  Babbit,  seen  from  below. 

;la  (=  Vestige  of  C< 
pula;  St.  Sternum. 


Cl.  Clavicle;  Cor.  Coracoid  process  of  Scapula  (=  Vestige  of  Coracoid);  Gl.  Glenoid 
cavity;  B.  Eibs;  Sc. 


together  under  the  terms  homology  and  analogy  (or  homoplasy) 
to  which  we  shall  next  refer. 

We  have  already  had  occasion  to  point  out  that  all  living 
organisms  are  more  or  less  perfectly  adapted  to  the  conditions  under 
which  they  exist,  and  are  accordingly  provided  with  organs  suitable 
for  their  various  requirements.  Thus  all  typical  vertebrates  have 
organs  of  locomotion  formed  from  two  pairs  of  limbs,  but  these 
differ  very  greatly  in  form  and  structure  in  accordance  with  their 
adaptation  to  very  diverse  conditions  of  life.  In  terrestrial 
vertebrates — amphibians,  reptiles  and  mammals — both  pairs  of 
limbs  usually  take  the  form  of  ambulatory  legs,  adapted  for 
moving  the  body  on  dry  land.  In  birds  the  hind  limbs  are 
generally  used  for  the  same  purpose  and  similarly  constructed,  but 


236        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

the  fore  limbs  are  modified  to  form  wings  for  flying  in  the  air ; 
and  the  same  is  true  of  the  fore  limbs  of  bats  and  of  the  extinct 


FIG.  91. — The    Duck-billed  Platypus  (Ornitliorliynclius   anatinus).     (From 
British  Museum  Guide.) 

flying  reptiles  known  as  Pterosauria  (pterodactyls).  The  Cetacea 
(whales  and  porpoises),  the  Sirenia  (dugongs  and  manatees)  and 
the  seals,  amongst  mammals ;  and  the  turtles  and  extinct 


FIG.  92.— The  Echidna  or  Spiny  Anteater  (Echidna  aculeata}.    (From  British 

Museum  Guide.) 

Ichthyosauria  and  Plesiosauria  amongst  reptiles,  on  the  other 
hand,  have  limbs  which  take  the  form  of  paddles  and  are 
specially  adapted  for  swimming  in  water. 

We  find,  then,  three  chief  types  of  locomotor  organs  amongst 


LIMBS  OF  VERTEBRATES 


237 


the  air-breathing  vertebrates :  ambulatory  legs,  wings  and  ^ 
paddles,  employed  in  totally  different  methods  of  locomotion  and 
differing  widely  from  one  another  in  general  form  and  appear- 
ance. Yet  when  we  come  to  examine  these  organs  closely  we  very 
soon  discover  the  remarkable  fact  that  all  are  constructed  on 
essentially  the  same  plan ;  all  belong  to  what  is  known  as  the 
pentadactyl  or  five-fingered  type  of  appendage. 

We  ourselves  retain  this  type  of  limb-structure  in  a  compara- 
tively primitive  condition  (Fig.  94),  although  our  fore  limbs  have 
taken  on  new  functions  as  organs  of  prehension.  The  fore  limb 


R. 


FIG.  93. — Shoulder  Girdle  and  part  of  Sternum  of  Ornithorhynchus,  seen 

from  below. 

CL  Clavicle ;  Cor.  Coracoid  ;  Ep.  C.  Epicoracoid ;  Gl.  Glenoid  cavity  ;  Int.  Interclavicle ; 
E.  Kibs ;  Sc.  Scapula ;  St.  Sternum. 

consists  of  arm,  forearm,  wrist  and  hand  (manus)  with  five 
fingers  ;  the  hind  limb  of  thigh,  leg,  ankle  and  foot  (pes)  with  five 
toes.  Each  segment  of  the  limb  is  supported  by  an  internal 
skeleton  of  bone  which  is  clothed  with  muscle  and  skin.  The  bones 
are  articulated  with  one  another  at  the  joints  and  moved  like  so 
many  levers  by  the  contraction  of  the  muscles  attached  to  them. 
The  bone  of  the  arm  is  the  humerus  and  the  corresponding  bone  of 
the  thigh  is  the  femur.  In  the  forearm  we  find  the  radius  and 
ulna  and  these  are  represented  in  the  leg  by  the  tibia  and  fibula. 
The  wrist  is  made  up  of  a  number  of  small  carpal  bones  and  the 
ankle  of  a  number  of  more  or  less  similar  tar  sals.  These  small 
bones  are  arranged  mainly  in  two  rows,  and  the  distal  or  far  row 


238       OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

affords  support  to  the  five  metacarpal  bones  in  the  palm  of  the 
hand  or  the  five  corresponding  metatarsal  bones  in  the  foot.  Each 
metacarpal  or  metatarsal  bone  in  turn  forms  the  support  of  a 
series  of  phalanges  which  constitute  the  skeleton  of  the  fingers  or 
toes.  The  thumb  and  great  toe  have  each  two  phalanges  and 
each  of  the  other  digits  has  three. 

When  the  human  arm  is  stretched  out  at  right  angles  to  the 
long  axis  of  the  body,  with  the  thumb  towards  the  head,  it  is  in 
the  primitive  position  of  the  pentadactyl  limb.  The  thumb  and 
the  radius  then  lie  on  what  is  called  the  preaxial  or  anterior  border, 


• 


FIG.  94. — Skeleton  of  the  fore  Limb  (A)  and  hind  Limb  (B)  of  Man,  showing 
the  pentadactyl  Structure.     (From  photographs.) 

c.  carpals;   fe.  femur;  fi.  fibula:  hum.  humerus;   m.c.  metacarpals;  mt.  metatarsals; 
ph.  phalanges ;  r.  radius  ;  t.  tarsals  ;  ti.  tibia ;  ul.  ulna ;  I — V,  digits. 

the  little  finger  and  the  ulna  on  the  postaxial  or  posterior  border 
of  the  limb.  For  convenience  of  comparison  with  other  types 
the  digits  are  numbered  from  in  front  backwards,  the  thumb 
being  No.  I  and  the  little  finger  No.  V.  Similarly  in  the  hind 
limb,  which  in  man  has  become  permanently  twisted  out  of  its 
primitive  position,  it  is  easy  to  show  that  the  great  toe  is  really 
the  preaxial  digit  (No.  I)  and  the  little  toe  the  postaxial  (No.  V). 
In  Fig.  94  both  limbs  are  represented  in  the  primitive  position. 

Starting  from  the  primitive  pentadactyl  type  let  us  next  inquire 
more  closely  how  different  groups  of  air-breathing  vertebrates  have 
solved  the  problems  of  locomotion  on  land,  in  air  and  in  water. 

In   considering  the  modifications   of  the   limbs   adapted   for 


LIMBS   OF   VERTEBRATES 


239 


locomotion  on  dry  land  we  may  conveniently  confine  our  attention 
to  the  hoofed  mammals  or  Ungulata.  In  the  elephant  the  limbs 
remain,  as  in  man,  in  a  primitive  condition,  with  five  well 
developed  digits  in  each.  The  entire  limb  is  stout  and  massive 
and  the  foot1  remains  very  short  and  affords  a  broad  support  for 


FIG.  95. — Skeletons  of  Man  and  Horse,  photographed  from  a  group  in*  the 
Natural  History  Museum.  (From  Lankester's  "  Extinct  Animals.") 

E,  Elbow  bone  (olecranon  process  of  ulna) ;  H,  Heel  bone  in  ankle  (the  hock  of  the 
horse) ;  K,  Knee  joint  (the  stifle  of  the  horse) ;  P,  Hip  bone ;  Sh,  Shoulder  bone 
(scapula) ;  T,  Vertebrae  of  tail  (coccyx  in  man) ;  W,  Wrist  or  carpus  (the  so-called 
knee  in  the  horse's  fore  leg). 

the  heavy  body.     This  type  of  limb  is  somewhat  clumsy  and  not 
adapted  for  very  rapid  locomotion. 

In  the  more  typical  ungulates  we  always  find  a  reduction  in  the 
number  of  digits,  accompanying  an  uplifting  of  the  hinder  part 
of  the  foot  from  the  ground,  until  finally  the  animal  comes  to 

1  The  term  foot  in  the  case  of  quadrupeds  is  commonly  used  to  include  both 
izmniis  and  pes, 


240        OUTLINES   OF    EVOLUTIONARY   BIOLOGY 

walk  and  rest  on  the  extreme  tips  of  one  or  two  toes  only.  The 
assumption  of  this  "  unguligrade  "  condition  is  accompanied  by 
elongation  of  the  limb  and  especially  of  the  remaining 
metapodial  bones  (metacarpals  and  metatarsals),  so  that  the  wrist 
and  ankle  in  the  horse,  for  example,  are  uplifted  high  above  the 
ground  and  form  the  so-called  knee  and  hock  respectively  (Fig.  95). 


FIG.  96.— Skeleton  of  (A)  the  right  fore  Foot  and  (B)  the  right  hind  Foot 
of  a  Tapir,  X  £.     (From  photographs.) 

c.  carpals ;  m.c.  metacarpals  ;  m.t.  metatarsals ;  ph.  phalanges ;  t,  tarsals ;  II — V,  digits. 

It  is  also  accompanied  by  a  reduction  of  the  ulna  in  the  fore  limb 
and  of  the  fibula  in  the  hind  limb  to  mere  vestiges.  Thus  the 
entire  limb  becomes  long  and  slender  and  adapted  for  rapid 
locomotion  by  running  on  hard  open  ground. 

The  disappearance  of  digits  from  the  manus  and  pes  in  the 

Mammalia  follows  a  very  simple  law.     The  first  to  disappear  is 

^  always  the  preaxial  digit  (No.  I),  which  i&  the  shortest  of  the 

series    (having   only  two   phalanges  while  the  remainder  have 


LIMBS  OF  VEETEBRATES 


241 


JT- 


—77 


ff- 


three  each)  and  consequently  the  first  to  leave  the  ground  as 
the  hinder  part  of  the  foot  is  uplifted.  The  next  to  go  is  the 
postaxial  digit  (No.  V)  from  the  opposite  side,  the  next  No.  II 
and  the  next  No.  IV. 

In  some  ungulates  the  long  axis  of  the  foot  passes  through 
the  middle  digit  (No.  Ill)  and  in  others  between  the  third  and 
fourth  digits.  The  true  ungulates  are 
accordingly  divided  into  two  series,  odd- 
toed  or  perissodactyl  and  even-toed  or 
artiodactyl. 

To  the  former  belong  the  tapir 
(Fig.  96),  with  four  digits  in  the  fore  foot 
and  three  in  the  hind ;  the  rhinoceros, 
with  four  or  three  digits  in  the  fore  foot 
and  three  in  the  hind,  and  the  horse 
(Fig.  97),  with  only 'the  middle  (third) 
digit  remaining  in  both  manus  and  pes, 
but  with  vestiges  of  Nos.  II  and  IV  in 
the  functionless  "  splint-bones  "  which 
lie  alongside  the  greatly  elongated  meta- 
podial  (=  metatarsal  or  metacarpal)  of 
No.  III. 

In  the  artiodactyl  series  (Fig.  98)  we 
find  the  hippopotamus,  with  four  well- 
developed  toes  on  each  foot ;  the  pig, 
with  four  toes  on  each  foot  but  the  two 
outer  ones  greatly  reduced;  the  deer, 
with  the  two  outer  digits  still  further 
reduced ;  the  sheep  and  oxen,  with  the 
second  and  fifth  digits  reduced  to  small 
nodules  of  bone,  and  the  camels  and 
llamas,  in  which  all  traces  of  digits  other 
than  the  third  and  fourth  have  completely  disappeared.  The 
gradual  reduction  of  the  number  of  digits  in  the  artiodactyl  series  is 
accompanied  by  fusion  of  the  two  remaining  metapodials  to  form 
a  single"  cannon  bone  "  (well  shown  in  Fig.  98,  deer  and  camel), 
whereby  the  now  greatly  elongated  and  uplifted  foot  acquires 
much  greater  strength  and  rigidity. 

It  is  very  easy  to  see  that  all  the  different  varieties  of 
perissodactyl  and  artiodactyl  limbs  are  modifications  of  one 
and  the  same  primitive  pentadactyl  type.  We  must  next 


FIG.  97. — Fore  and  hind 
Feet  of  a  Horse,  show- 
ing reduction  to  a  single 
functional  Digit  (No. 
Ill)  with  Vestiges  of 
two  others  (II  and  IV) 
in  the  form  of  splint 
bones,  X  &.  (From 
Lull,  after  Marsh.) 


B. 


242        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

consider  the  way  in  which  this  type  has  been  adapted  to  form 
organs  of  flight  in  different  vertebrate  groups. 

The  wing  of  the  extinct    pterodactyls    (Fig.  99)  was    iormed 


by  an  expansion  of  the  slim  of  the  body  stretched  between  the 
fore  and  hind  limbs,  and  the  phalanges  of  the  little  finger  were 
enormously  elongated  so  as  to  aid  in  the  support  of  its  anterior 


LIMBS   OF   VERTEBRATES  £43 

margin.     The  surface    of    the    flying   membrane    was  probably 
covered  with  scales.       In  the  bats  (Fig.  99)  four  of  the  digits 


of  the  hand,  Nos.  II— V,  are  elongated  and  take  part  in  the 
support  of  the  flying  membrane,  the  surface  of  which  is  either 
naked  or  covered  with  fine  hair.  In  the  birds  (Fig.  99)  the 

R  2 


244        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

flying  membrane  is  formed  chiefly  of  feathers,  and  the  distal 
part  of  the  wing  is  supported  mainly  by  the  second  digit ; 
the  first  and  third  digits  are  greatly  reduced,  and  the 
other  digits  are  absent.  In  all  these  different  types  of  wing 
the  skeleton  is  again  clearly  of  the  pentadactyl  type,  modified 
by  enlargement,  diminution  or  total  suppression  of  certain  of  its 
constituent  bones. 

In  the  paddles  of  the  modified  aquatic  mammals,  such  as  the 
sirenians,  seals  (Fig.  100)  and  whales  (Fig.  101),  the  whole  limb 


FIG.  100. — Skeleton  of  right  fore  Foot  (A)  and  right  hind  Foot  (B)  of  a 
Seal  (Otaria  hookeri),  X  &.     (From  photographs.) 

c.  carpals ;  m.c.  metacarpals ;  m.t.  metatarsals ;  ph.  phalanges ;  t.  tarsals  ;  I — V,  digits. 

is  very  much  shortened  and  flattened  and  the  digits  are  enclosed  in 
a  common  integument  so  as  to  offer  a  greater  resistance  to  the 
water,  but  the  skeleton  still  reveals  the  essential  pentadactyl 
features,  and  the  same  is  true  of  those  aquatic  reptiles,  the 
turtles,  plesiosaurians  (Fig.  141)  and  ichthyosaurians  (Fig.  142). 
In  the  whales,  however,  a  curious  increase  in  the  number  of 
phalanges  (hyperphalangy)  may  often  be  observed,  and  in  the 
ichthyosaurians  supplementary  small  bones  are  developed  in 
such  positions  that  the  entire  skeleton  of  the  paddle  assumes 
the  form  of  a  mosaic  pavement. 


LIMBS  OF   VERTEBEATES 


245 


246        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

We  can  only  explain  the  occurrence  of  the  same  type  of 
skeleton— and  that  a  very  complex  type— in  all  these  different 
kinds  of  organs  of  locomotion  on  the  assumption  that  it  has  been 
inherited  from  common  ancestors.  We  cannot  believe  that  one 
and  the  same  type  of  skeleton  was  necessarily  the  most  suitable 
for  all  these  different  cases,  including  ambulatory  legs,  wings  and 
paddles,  and  was  therefore  specially  and  independently  created  for 
each.  We  must  conclude  rather  that  each  organism  receives  a 
certain  kind  of  material  by  inheritance  from  its  ancestors  and 
has  to  adapt  it  to  its  own  requirements  as  best  it  may ;  has,  in 
short,  to  cut  its  coat  according  to  its  cloth,  and,  whatever  the 


* 


S.I. 


FIG.  102. — A  swimming  Crab  (Portunm  depurator),  showing  jointed  ambu- 
latory appendages  and  also  swimming  appendages  (s.L).  (From  a 
photograph.) 

shape  of  the  coat  may   ultimately  develop  into,  the  cloth  will 
retain,  more  or  less  evidently,  traces  of  the  original  pattern. 

This  conclusion  is  greatly  strengthened  when  we  turn  to  other 
groups  of  animals  and  see  how  they  have  solved  the  same  problems 
with  the  aid  of  different  materials.  The  vertebrates  are  not  the 
only  animals  which  have  organs  of  locomotion  in  the  form  of 
jointed  appendages.  Many  members  of  the  great  group 
Arthropoda — insects,  crustaceans  and  spiders — have  ambulatory 
limbs  which  externally  bear  considerable  resemblance  to  those 
of  vertebrates.  If  we  examine  these  limbs,  however,  we  find  that 
they  are  constructed  upon  a  wholly  different  plan.  In  the  first 
place  the  skeleton  is  external  instead  of  internal,  and  is  composed 


HOMOLOGY  AND  ANALOGY         247 

of  the  hard  cuticle  secreted  by  the  epidermis,  there  being  no  true 
bone.  The  muscles  therefore  lie  inside  the  skeleton  instead  of 
outside.  The  number  and  arrangement  of  the  limb-segments, 
again,  is  totally  different,  an£  toere  is,  of  course,  no  pentadactyl 
structure  (compare  Fig.  102^*" 

Thus,  with  different  material  at  their  disposal,  the  arthropods 
have  solved  the  problem  of  constructing  an  ambulatory  appendage 
in  a  very  efficient  manner,  but  quite  differently  from  the 
vertebrates.  Many  of  thenwhave  also  solved  the  problem  of 
locomotion  in  water  by  modification  of  certain  of  the  limbs  to 
form  paddles,  as  in  the  case  of  the  sjafmming  crab  (Fig.  102). 
Many  insects,  on  the  otner  hand,  haye  acquired  the  power  of 
flight  by  means  offings,  which,  though  bearing  some  external 
resemblance  to  those  of  vertebrates,  are  totally  different  in 
structure  and  origin  not  only  from  the  latter  but  also  from  the 
other  appendages  of  the  arthropods  themselves. 

We  are  now  in  a  position  to  define  the  meaning  of  the  terms 
homology  and  analogy  as  used  by  biologists.  Homologous 
organs  are  such  as  have  the  same  essential  structure, 
which  they  owe  to  inheritance  from  common  ancestors, 
though  they  may  be  very  differently  modified  in  adaptation  to 
different  functions.  The  pentadactyl  limbs  of  air-breathing 
vertebrates,  however  much  theylnay"  differ  amongst  themselves, 
are  all  homologous  organs  in  so  far  as  their  essential  pentadactyl 
structure  is  concerned. 

Analogous  or  homoplastic  organs,  on  the  other  hand,  bear  only 
a  superficial  resemblance  to  one  another,  which  they  owe  not  to 
common  ancestry  but  to  adaptation  of  fundamentally  different 
structures  along  similar  lines  for  similar  functions.  The 
ambulatory  appendages  of  arthropods  and  vertebrates  are 
analogous  but  not  homologous  organs,  so  also  are  the  wings  of 
birds  and  insects. 

The  evolutionary  process  by  which  analogous  but  not 
homologous  structures  have  come  to  resemble  one  another  is 
sometimes  spoken  of  as  convergence,  and  the  result  may  be  looked 
upon  as  an  illustration  of  the  general  principle  that  similar  causes 
tend  to  produce  similar  effects.  The  necessity  for  the  adaptation 
of  different  organs  and  organisms  to  the  same  environment  and 
the  same  mode  of  life  results  in  a  superficial  resemblance  between 
the  organs  and  organisms  thus  adapted. 

One   of  the  most  familiar   examples-  of  convergent  evolution 


248        OUTLINES   OF   E VOLUTION AEY   BIOLOGY 


is  afforded  by  the  whales,  dolphins  and  porpoises  (Figs.  101 
and  161)  with  their  extraordinary  external  ressmblance  to 
fishes.  The  fore  limbs,  as  we  have  already  seen,  are  modified 
to  form  paddles,  but  they  retain  the  pentadactyl  structure  and 
are  thus  totally  different  from  the  fins  of  fishes,  which  have 
never  reached  the  pentadactyl  stage.  The  hind  limbs  have 
completely  disappeared,  but  vestiges  of  the  limb  bones  and  their 
supporting  girdle,  which  in  some  cases  are  found  buried  deeply 
beneath  the  skin  in  the  pelvic  region  (Fig.  101,  f,p),  still  bear 
witness  to  their  former  presence.  The  powerful  tail  fin  has 
obviously  no  real  relationship  to  the  tail  fin  of  a  fish,  for  it 

lies  horizontally  instead  of 
vertically.  There  are,  of 
course,  no  gills,  as  in  fishes, 
but  the  animal  comes  to 
the  surface  to  breathe  air 
by  means  of  lungs. 

Not  only  is  the  whale 
1  not  a  fish,  but  it  belongs 
uto  the  group  of  vertebrates 
nmost  remote  from  fishes. 
It  is  a  warm  -  blooded 
mammal,  suckling  its 
young  and  exhibiting  other 
characteristically  mamma- 
lian features.  It  has  seven 
"prYJffiil  vftrtftbrgp, a  number 
which  is  curiously  constant 
throughout  the  mammalian  series,  but  owing  to  the  extreme 
shortness  of  the  neck  these  vertebrae  are  all  crushed  together  to 
form  practically  a  single  bone  (Fig.  103).  In  the  giraffe  we  also 
s  find  seven  cervical  vertebrae,  but  they  are  all  greatly  elongated 
m  accordance  with  the  enormous  length  of  the  neck  (Fig.  104). 
On  the  hypothesis  of  special  creation  we  should  certainly  havo 
expected  the  whale  to  have  fewer  vertebrae  in  its  neck  than  the 
giraffe,  and  we  can  only  suppose  that  the  number  seven  has 
been  inherited  from  some  common  mammalian  ancestor.  The 
resemblance  of  the  whale  to  the  fish,  in  short,  is  simply  due  to  the 
fact  that  both  have  acquired  the  external  form  best  adapted  for  an 
active  aquatic  life. 

The  wings  of  pterodactyls,  bats  and  birds   are  equally  good 


PIG.  103. — The  seven  cervical  Vertebra 
of  a  Whale,  fused  together  in  one 
Mass.  (From  Reynolds'  "  Vertebrate 
Skeleton.") 


CONVERGENT  EVOLUTION 


249 


examples  of   convergent  evolution  within  much  narrower  limits. 
They  can  only  be  regarded  as  homologous  to  the  extent  of  all  being 


FIG.  104. — Skeleton  of  the  Giraffe,  showing  the  seven  separate  and  greatly 
elongated  cervical  Vertebrae.     (From  Brehm's  "  Thierleben.") 

pentadactyl.    They  do  not  owe  their  special  characters  as  wings 
to  descent  from  a  common  ancestral  winged  form,  but  have  been 


250       OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

evolved  independently  from  more  primitive  pentadactyl  types  of 
limb. 

We  find  analogous  cases  in  other  vertebrate  groups.  Several 
of  the  lizards,  such  as  the  so-called  slow  worm  or  blind  worm 
(Fig.  105)  and  the  "  glass  snakes  "  (Ophisaurus),  have,  by  loss 
of  their  limbs,  come  so  closely  to  resemble  snakes  as  to  be 


FlG.  105. — The  Slow  Worm,  Anguis  fragilis,  X  £.     (From  a  photograph.) 

indistinguishable  by  most  people.  The  Coeciliidse  (Fig.  106) 
amongst  amphibians,  the  Amphisbsenidae  amongst  lizards  and 
the  Typhlopidse  amongst  snakes  have  all  adapted  themselves  to  a 
burrowing  underground  life,  like  the  earthworms.  Their  limbs 
have  completely  disappeared,  the  body  has  become  cylindrical  and 
worm-like  and  they  have  lost  the  use  of  their  eyes,  but  though  they 


FIG.  106. — A  worm-like,  limbless  Amphibian,  Urceotyphltts  africanus.  (From 
British  Museum  Guide.) 

have  all  come  to  resemble  one  another  closely  as  the  result  of 
convergent  evolution  they  are  not  in  reality  at  all  nearly  related. 
The  loss  of  limbs,  the  assumption  of  the  worm-like  form,  &c., 
have  taken  place  quite  independently  in  each  group. 

The  mammalian  fauna  of  Australia  consists  mainly  of  members 
of  the  single  order  Marsupialia  or  pouched  mammals,  but 
different  representatives  of  this  order  have  come,  by  convergence, 
to  resemble  closely  members  of  widely  different  orders  of 


CONVERGENT   EVOLUTION  251 

mammals  found  in  other  parts  of  the  world.  The  so-called 
marsupial  wolf  of  Tasmania  (Thylacinus)  closely  resembles  a 
typical  carnivore  in  its  habits  and  general  structure.  Its  teeth 
are  adapted  for  a  predaceous  life,  and  the  entire  skull  (Fig.  107,  B), 
with  its  dentition,  bears  an  extraordinary  general  resemblance  to 
that  of  a  dog  (Fig.  107,  A),  being  distinguishable  only  by  details 
of  structure  which  would  hardly  be  noticed  except  by  an 


FIG.  107.— A.  Skull  of  Dog,  side  view  ;  B.  Skull  of  Thylacine,  side  view. 
(From  photographs.) 

anatomist.  These  details,  however,  are  quite  sufficient  to  show 
that  there  is  in  reality  no  close  relationship  between  the  two. 
Thus  the  dog  (Fig.  108,  A,  AI)  has  in  each  upper  jaw  three 
incisor  teeth  (i.1 — i.3),  one  canine  (c.),  four  premolars  (p.m.1 — p.m.4) 
and  two  molars  (m.1 — m.2) ;  and  the  dentition  of  the  lower  jaw  is 
the  same  except  for  the  presence  of  a  third  molar  (m.3),  which  is, 
however,  in  a  vestigial  condition.  The  thylacine,  on  the  other 
hand  (Fig.  108,  B,  BI),  has  four  incisors  (i.1— i.4)  in  the  upper  and 


252        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


FIG.  108.— A,  Ai,  Skull  and  Mandible  of  Dog ;  B,  BI,  Skull  and  Mandible 
of  Thylacine,  to  show  the  arrangement  of  the  teeth,  &c.  (From 
photographs.) 

i. l. — i.4,  incisors  of  upper  jaw;i.i — i.3,  incisors  of  lower  jaw;  c.,  canine;  p.m.i — p.m. 4, 
prernolars  of  upper  jaw  ;  p.m.i — p.m.4,  premolars  of  lower  jaw ;  m.1 — rn.4,  niolai 
of  upper  jaw ;  m.i — m.4,  molars  of  lower  jaw. 


CONVEKGENT   EVOLUTION  253 

ihree   (i.i — i.s)   in    the   lower   jaw ;  and  one  canine   (c.),  three 

)remolars  (p.ni.i — 3)  and  four  molars  (m.i — 4)  in  each  jaw.      The 

,hylacine  skull  is  further  distinguished  from  that  of  the  dog  by 

he  presence  of  large  vacuities  in  the  hinder  part  of  the  bony 

palate  and  by  the  strongly  marked  inflection  of  the  angles  of  the 

mandibles.     These  two  characters,  both  of  which  are  shown  in 

he  illustrations,  as  well  as  the  arrangement  of  the  cheek  teeth 

and  other  minor  features  which  it  is  unnecessary  to  specify,  are 

undamental  peculiarities  of  the  great  group  Marsupialia  and  at 

>nce  indicate  the  true  affinities  of  Thylacinus. 

The  case  of  the  Australian  marsupial  mole,  Notoryctes,  is 
qually  striking.  The  powerfully  built  digging  limbs,  the  soft, 
lose  fur,  the  absence  of  external  ears  and  the  loss  of  sight,  as 
well  as  the  general  shape  of  the  body,  all  in  adaptation  to  its 
•urrowing  habits,  cause  it  to  assume  a  wonderfully  mole-like 
spect,  while  in  reality  it  comes  nowhere  near  the  true  moles  as 
egards  genetic  relationship. 

Perhaps  the  most  remarkable  of  all  known  cases  of  convergent 
volution,  however,  is  met  with  in  the  Ungulata.  We  have 
Iready  seen  that  amongst  the  typical  ungulate  mammals  of  the 
>resent  day  we  can  distinguish  two  series,  odd-toed  or  perisso- 
dactyl,  and  even-toed  or  artiodactyl.  In  the  odd-toed  series  the 
reduction  of  the  digits  culminates  in  the  horse,  with  a  single 
perfect  digit  in  each  foot  and  vestiges  of  two  others  in  the  form 
of  splint-bones.  This  extreme  modification  of  the  unguligrade 
type  is  so  peculiar  that  it  is  difficult  to  believe  that  precisely  the 
same  line  of  evolution  has  been  followed  independently  by  two 
different  groups  of  animals.  Yet  such  appears  to  be  actually 
the  case.  There  is  an  extinct  group  of  ungulates  known  as  the 
Litopterna,  whose  remains  have  been  described  by  Ameghino 
from  Tertiary  beds  of  Patagonia.  The  small  size  of  the 
brain-cavity,  the  characters  of  the  dentition,  cervical  vertebrse, 
carpus  and  tarsus,  indicate  that  they  were  more  primitive  forms 
than  the  true  Perissodactyla,  and,  as  Dr.  Smith  Woodward 
observes,  they  reached  their  maximum  of  specialization  at  an 
earlier  period  than  the  latter.  We  find  amongst  them  forms 
(Theosodon,  Fig.  109,  A,  B)  with  three  well-developed  toes  as  in 
the  rhinoceros,  forms  (Proterotherium,  Fig.  109,  C)  with  one  well- 
developed  toe  and  two  small  ones  as  in  some  of  the  extinct 
ancestors  of  the  horse,  and  forms  (Thoatherium,  Fig.  109,  D,  E) 
with  a  single  toe  as  in  existing  horses  ;  and  in  all  cases  the  axis 


254        OQTLINES   OF   EVOLUTIONABY  BIOLOGY 


.  109.— Feet  of  Ungulates 
belonging  to  the  extinct 
group  Litopterna,  from 
Lower  Tertiary  deposits  of 
Patagonia.  (From  Smith 
Woodward's  "Vertebrate 
Palaeontology,"  after 
Ameghino. ) 


y  m 


A,  B,  Theosodon  lydekkeri;  right  fore  and  hind  feet,  x  J. 

C,  Proterotherium  intermixtum  ;  right  fore  foot,  x  J. 

D,  E,  Thoatherium  crepidatum;  left  fore  and  hind  feet,  x 


CONVEKGENCE   AND   CHANGE    OF   FUNCTION     255 

of  the  foot  passes  down  the  middle  of  the  third  digit.  If  we 
compare  the  foot  of  Thoatherium  (Fig.  109,  D,  E)  with  that  of  a 
horse  (Fig.  97)  it  is  hard  indeed  to  imagine  that  the  animals 
possessing  such  closely  similar  and  highly  specialized  limbs  are 
not  nearly  related  to  one  another.  Expert  palaeontologists, 
however,  tell  us  that  they  are  not,  and  we  must  believe  that  the 
resemblance  is  due  entirely  to  adaptation  to  a  similar .  mode  of 
life  in  the  two  cases.  We  shall  have  something  to  say  as  to  how 
this  adaptation  was  brought  about  in  the.  case  of  the  horse  when 
we  come  to  discuss  the  ancestral  history  of  that  animal  in 
Chapter  XIX. 

It  will  be  readily  understood  from  the  examples  which  we  have 
been  considering  that  the  phenomena  of  Convergence  provide 
many  pitfalls  for  the  unwary  biologist,  and  have  led  to  many 
other  mistakes  in  classification  besides  the  popular  error  of 
placing  the  whales  amongst  the  fishes.  "  We  have  already  noticed 
how  the  limbs  of  arthropods  have  come  to  bear  a  superficial 
resemblance  to  those  of  vertebrates,  though  so  absolutely  different 
in  their  essential  structure  that  no  anatomist  would  dream  of 
regarding  them  as  homologous  organs.  Many  aquatic  arthropods, 
belonging  to  the  class  Crustacea,  Jijse  the  lobsters,  cray-fishes 
and  crabs,  breathe  by  means  of  gills  which  bear  a  superficial 
resemblance  to  those^of  fishes,  but  are  again  by  no  means 
homologous  structures,  and  there  are  other  resemblances  between 
arthropods  and  vertebrates,  due  probably  to  convergence,  which 
have  led  more  than  one  observer  to  conclude  that  vertebrates  are 
descended  from  arthropod  ancestors,  a  conclusion  which  is  by  no 
means  justified  by  the  facts. 

We  may  now  further  consider  the  process  known  as 


of^ function/'  in  which  an  organ  primarily  adapted  and  used  for 
one  purpose  takes  on  a  new  and  altogether  -different  duty  and 
becomes  modified  accordingly.  The  lungs  of  air-breathing 
vertebrates,  for  example,  are  generally  believed  to  be  homologous 
with  the  swim-bladder  of  fishes,  for  both  arise  in  the  same  way  as 
outgrowths  of  the  front  part  of  the  alimentary  canal,  in  the 
region  of  the  throat.  The  swim-bladder  of  a  fish  is  a  hydrostatic 
organ  ;  it  is  filled  with  gas,  the  amount  of  which  can  be  regulated 
by  suitable  means,  and  assists  the  animal  in  maintaining  its 
proper  position  in  the  watejc-J  In  the  dipnoan  fishes,  such  as 
the  Australian  mud-fish  (Neoceratodus,  Fig.  110),  the  South 
American  Lepidosiren  and  the  African  Protopterus,  the  walls  of 


256        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

the  swim-bladder  have  become  highly  vascular  and  it  serves  as 
an  organ  of  respiration.  Air  is  taken  into  it  through  the  mouth, 
so  that  the  blood  circulating  in  its  wall  is  oxygenated,  and  the 
arrangement  of  the  heart  and  blood-vessels  has  become  modified 
accordingly.  Functional  gills,  however,  are  still  present. 

In  the  amphibians  the  swim-bladder  has  become  completely 
converted  into  a  pair  of  lungs,  and  in  this  way  was  rendered 
possible  that  great  step  in  the  evolution  of  the  vertebrates,  the 
migration  from  water  to  land.  Thus  in  the  air-breathing  verte- 
brates the  gills  have  been  entirely  supplanted  and  replaced 
functionally  by  the  lungs,  which  are  still  to  be  regarded  as 
homologous  with,  or  morphologically  equivalent  to,  the  air- 
bladder  of  fishes. 

The  hand  of  man  affords  another  beautiful  example  of  change 


FIG.  110. — The  Australian  Mud-Fish,  Neoceratodus  forsteri,  greatly  reduced. 
(From  British  Museum  Guide.) 

of  function — from  locomotion  to  prehension — but  it  has  under- 
gone surprisingly  little  structural  modification  in  the  process. 
The  proboscis  of  the  elephant  is  also  a  very  efficient  organ  of 
prehension,  but  it  is  formed  by  enormous  elongation  of  the  nose, 
which  is  primarily  an  organ  for  conveying  air  to  and  from  the 
olfactory  organs  and  lungs.  Still  more  remarkable  is  the  con- 
version of  muscle  fibres  in  the  torpedo  and  the  electric  eel 
(Gymnotus)  into  powerful  electric  batteries,  capable  of  giving 
severe  shocks  and  therefore  valuable  as  weapons  of  offence  and 
defence.  In  this  case  the  change  of  function  is  accompanied  by 
very  profound  modifications  in  structure. 

Wherever  we  turn  we  find  that  novel  requirements  are  met,  not 
by  the  sudden  creation  of  new  organs,  but  by  the  gradual  modifi- 
cation of  old  ones.  As  we  have  already  said,  the  "organism  has 
to  do  its  best  with  the  material  which  it  has  inherited  from  its 
ancestors  ;  and  yet  the  power  of  living  protoplasm  to  meet  every 
new  requirement  by  a  suitable  modification  of  bodily  structure 


VESTIGIAL   ORGANS  257 

appears  to  be  almost  unlimited.  We  must  remember,  however, 
that  such  modifications,  in  a  state  of  nature  at  any  rate,  only 
take  place  very  slowly  and  gradually. 

y  Traces  or  vestiges  of  organs  which  have  ceased  to  be  of  any 
use  to  their  possessors  persist  with  astonishing  pertinacity  in 
many  organisms.  Evidently  their  complete  removal  must  be 
an  extremely  slow  process.  We  have  already  noticed  the  per- 
sistence of  vestiges  of  the  pelvic  girdle  and  leg  bones  in  the 
whale  (Fig.  101),  of  the  reduced  metapodials  or  splint  bones  in 


j  "x   y  ~r    A,  -^*    '~~\~-*f 

FIG.  111. — A  Xf\|^  Zcalamf-  Kiwi,  X  £.     (Frcm  a  photograph  of  a  stuffed 

.-pecimen.) 

the  feet  of  the  horse  (Fig.  97),  and  of  the  coracoid  in  the  typical 
mammalian  shoulder  girdle  (Fig.  90).  Unless  we  are  to  believe  that 
such  structures  have  been  put  where  they  are  on  purpose  to 
mislead  us  we  cannot  possibly  explain  their  occurrence  on  the 
theory  of  special  creation.  They  are,  at  any  rate  in  many 
cases,  perfectly  useless  to  their  possessors,  and  the  only  rational 
way  of  accounting  for  their  presence  is  by  supposing  them 
to  be  inherited  from  remote  ancestors  in  which  they  were 
functional. 

We  may  now  briefly  notice  a  few  other  instances   of    such 
B.  s 


258        OUTLINES   OF   EVOLUTIONAEY   BIOLOGY 


vestigial  structures.  A  characteristic  feature  of  oceanic  islands 
is  the  presence  on  them  of  birds  which  have  more  or  less  lost  the 
power  of  flight.  Such  were  the  dodo  of  Mauritius,  the  solitaire 
of  Eodriguez,  and  the  gigantic  moas  of  New  Zealand.  All  these 
forms  are  now  extinct,  but  we  still  find  in  New  Zealand  several 
flightless  birds  surviving.  One  of  the  most  interesting  is  the 
kiwi  or  Apteryx  (Fig.  Ill),  a  moderate-sized  bird  of  nocturnal 

habits,  now  rapidly  be- 
coming exterminated. 
The  whole  body  is  covered 
with  coarse,  hair  -  like 
plumage  and  externally 
shows  no  trace  of  wings. 
There  is,  however,  a 
minute  remnant  of  a 
wing  present  on  each 
side,  completely  con- 
cealed in  the  plumage 
and  entirely  f  unctionless 
as  an  organ  of  flight. 
Yet  it  still  possesses  the 
pentadactyl  skeleton, 
though  in  a  greatly 
reluced  condition  (Fig. 
112).  It  is  said  that 
when  the  kiwi  goes  to 
sleep  it  still  endeavours 
to  tuck  its  long  beak 
under  its  wing  after  the 
FIG.  112. — Skeleton  of  a  Kiwi,  showing  tlie  manner  of  other  birds, 
vestigial  Wing  Bones,  behind  which  a  T  ,,  PYKr.r4  rnnpci  fn 
piece  of  Black  Paper  has  been  placed,  ln  tne  extmct  moas>  to 
X  J.  (From  a  photograph.)  which  the  kiwis  are  per- 

haps  related,   even    the 

last  vestiges  of  the  wings  seem  to  have  disappeared,  for  amongst 
the  enormous  quantities  of  the  remains  of  these  gigantic  birds 
which  occur  in  New  Zealand  no  wing  bones  have  ever  been  found. 
There  is  very  strong  reason  to  believe  that  the  remote  ancestors 
of  existing  vertebrates  possessed  an  additional  pair  oij  eyes  on 
the  top  of  the  head,  behind  the  still  existing  lEteraT^yes. 
Traces  of  this  second  pair — the  pineal  or  parietal  eyes — are  yet 
to  be  found  in  lampreys  and  lizards,  and  especially  in  that 


VESTIGIAL   ORGANS 


25CJ 


remarkable  New  Zealand  reptile  the  tuatara  (Sphenodonpunctatus). 
This  animal  (Fig.  113)  is  usually  regarded  as  the  oldest  surviving 


•~i-'.>£?=*  - 


<• 


FIG.  1 13.— The  New  Zealand  Tuatara,  X  £.     (Drawn  by  Miss  V.  E.  Bendy 
from  a  model  and  photographs.) 

type  of  terrestrial  vertebrate,  the  order  to  which  it  belongs  (the 
Rhynchocephalia)  dating  back  to  the  close  of  the  Palaeozoic  period 
of  the  earth's  history  (Permian  epoch). 

In  the  adult  tuatara  there  is  still  a  fairly  well  developed  pineal 


FIG.  114.— Section  through  the  Pineal  Eye  of  the  Tuatara,  X  68.     (From 

photograph.) 
le.,  lens ;  n.,  nerve ;  ret.,  retina. 

eye  (Fig.  114),  with  retina,  lens  and  nerve,  but  it  is  only  about 
inch  in    diameter    and    lies   deeply   buried    beneath    the 

s  2 


260        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


skin  in  the  parietal  foramen  in  the  middle  of  the  roof  of  the 
skull.  It  may  to  some  extent  still  be  functional,  though  probably 
of  .far  less  importance  than  in  the  extinct  amphibia  and  reptiles 
of  the  Palaeozoic  and  Mesozoic  periods,  in  which  the  presence  of 
a  comparatively  large  parietal  foramen  indicates  that  the  eye  was 
better  developed.  In  the  case  of  the  tuatara  it  has  been  demon- 
strated by  anatomical  and  embryological  investigation  that  the 
remaining  pineal  eye  is  the  left-hand  member  of  the  original 
pair.  In  the  lampreys  it  appears  to  be  the  right-hand  member. 

Turning  now  to  the  frog,  we  sometimes  find  in  this  animal  a 
minute  light-coloured  spot  on  top  of  the  head  (Fig.  115),  between 
the  paired  eyes.  This  marks  the  position  of  a  little  sac-shaped 

vesicle  which  lies 
beneath  the  skin  and 
is  known  as  Stieda's 
organ, consisting  merely 
of  a  number  of  undiffer- 
entiated  cells  surround- 
ing a  central  cavity  and 
attached  to  the  end  of  a 
kind  of  stalk  (Fig.  116). 
The  vesicle  represents 
the  functionless  vestige 
of  a  pineal  eye  and  the 
stalk  probably  repre- 
sents its  disappearing 
nerve. 

In  the  birds  and  mammals  all  trace  of  eye-like  structure  has 
disappeared  from  this  region,  but  a  vestige  of  one  or  both  of  the 
pineal  eyes  is  probably  to  be  recognized  in  the  so-called  pineal 
gland  lying  on  the  roof  of  the  brain,  which  attained  such  celebrity 
in  the  seventeenth  century  owing  to  its  identification  by  the 
— philosopher  Descartes  as  the  seat  of  the  soul. 

In  studying  the  mammalian  de_niition  we  again  meet  with 
plenty  of  illustrations  of  vestigial  structures.  The  whale-bone 
whales  (Balaenidae)  have  no  teeth  in  the  adult  at  all,  but  in  the 
foatus  vestiges  of  teeth  can  be  found  imbedded  in  the  gums.  The 
young  Australian  duck-billed  Platypus  (Ornithorhynchus)  his 
vestigial  teeth  which  are  entirely  replaced  by  horny  pads  in  the 
adult.  The  third  molar  in  the  lower  jaw  of  the  dog  (Fig.  108, 
AI,  m.a) '  is,  as  we  have  already  seen,  evidently  in  a  vestigial 


FIG.  115. — Head  of  Frog  (Ran<t  temporaria}, 
showing  Stieda's  Organ  between  the  lateral 
Eyes.  (From  Studnicka,  after  Stieda.) 


VESTIGIAL   ORGANS  261 

condition  and  can  be  of  very  little  if  any  use  to  its  possessor, 
and  there  is  reason  to  fear  that  in  man  himself  the  teeth  are 
disappearing  a  good  deal  more  rapidly  than  we  could  wish, 
though  in  this  case  the  disappearance  seems  to  be  chiefly  due 
to  disease. 

A   better   example   of    a   vestigial    structure   in   man   is   the 
coccyx  at  the  hinder  end  of  the  vertebral  column  (Fig.  95),  which 
j/represents  the  last  remnant  of  the  ancestral  tail,  and  is  occasion- 
ally accompanied  by  vestiges  of  the  muscles  by  which  an  ordinary 
tail  is  moved.     The  hair  on  the  chest,  again,  is  a  vestige  of  the 


V 

FIG.  116. —  Section  through  the  vestigial  Pineal  Eye  of  the  Frog  (Tadpole) 

X  168.     (From  a  photograph.) 
epd.,  epidermis  ;  p.e.,  pineal  eye;  r.s.,  roof  of  skull ;  v.n.,  vestigial  stalk  and  nerve. 

hairy  coat  which  once  clothed  the  entire  human  body,  as  it  still 
does  that  of  the  apes. 

Examples  of  vestigial  organs  might  be  multiplied  to  an 
indefinite  extent,  but  enough  has  perhaps  been  said  to  show  that 
such  structures  are  of  very  common  occurrence  and  to  indicate 
their  value  as  evidences  of  organic  evolution.  They  occur,  of 
course,  not  only  in  the  adult  condition  but  also,  as  in  the  case  of 
the  embryonic  gill-slits  of  air-breathing  vertebrates,  at  earlier 
stages  ofaeveTopment,  but  such  cases  will  be  more  conveniently 
dealt  with  in  the  next  chapter. 

Closely  akin  to  the  occurrence  of  vestigial  structures  is  another 
phenomenon  known  as  atavism  or  reversion,  by  which  we  mean 


262    *   OUTLINES   OF   E VOLUTION AKY  BIOLOGY 

the  sudden  and  sporadic  reappearance  of  some  ancestral  structure 
which  had  either  been  completely  lost  or  very  greatly  reduced. 
The  elder  Pliny  has  placed  it  on  record  that  Caesar  the  Dictator 
had  a  horse  whose  fore  feet  were  like  those  of  a  man.  This 
statement  evidently  refers  to  a  more  or  less  complete  return  to 
the  ancestral  pentadactyl  condition.  Marsh  has  given  a  figure 
of  the  fore  foot  of  a  horse  in  which  the  second  digit  is  fairly 
well  developed,  with  three  perfect  phalanges,  though  not  so  large 
as  the  third,  while  the  first  is  represented  by  a  splint  bone,  and  a 
very  similar  case  is  exhibited  in  the  Natural  History  Museum  at 
South  Kensington. 

In  man  the  occasional  excessive  development  of  the  .canine 
teeth  is  regarded  as  due  to  reversion  to  an  ancestral  condition 
similar  to  that  of  the  anthropoid  apes,  in  which  the  canine  teeth 
are  very  large  and  used  as  weapons  The  occasional  occurrence 
in  man.  of  vestigial  tail  muscles,  and  even  of  a  short  tail,  may 
also  be  regarded  as  due  to  reversion.  Whether  or  not  all  such 
cases  are  capable  of  being  explained  on  Mendelian  principles,  as 
suggested  in  Chapter  XIV,  it  is  impossible  to  say,  but  this  does 
not  affect  their  importance  as  evidence  of  the  truth  of  the  theory 
of  organic  evolution. 

Although,  in  the  present  chapter,  we  have  so  far  drawn  our 
illustrations  entirely  from  the  animal  kingdom,  it  must  not  be 
supposed  that  the  same  general  principles  cannot  be  equally  well 
demonstrated  in  the  case  of  plants.  Indeed  the  whole  chapter 
might  be  re-written  entirely  from  the  botanical  point  of  view. 
Change  of  function  is  very  well  shown,  for  example,  in  the 
"  pitchers  "  of  Nepenthes  and  Sarracenia,  formed  from  modified 
leaves  and  serving  as  traps  for  catching  insects.  Convergent  evolu- 
tion is  illustrated  by  the  strong  superficial  resemblance  which  exists 
amongst  the  various  Alpine  cushion  plants  belonging  to  widely 
different  orders,  all  of  which  have  assumed  the  form  best  suited 
for  withstanding  the  peculiar  hardships  of  their  environment. 
The  leaves  of  the  parasitic  dodder,  reduced  to  tiny  scales  (Fig. 
184),  and  the  staminodes  or  functionless  stamens  of  many  flowers, 
again,  might  well  serve  as  examples  of  vestigial  structures. 
Considerations  of  space,  however,  forbid  us  to  pursue  this  fertile 
line  of  inquiry  any  further. 


CHAPTEK  XVIII 

Ontogeny  —The    recapitulation  hypothesis — Interpretation    of    the    onto- 
genetic  record — Palingeiietic  and  csenogenetic  characters. 

IN  dealing  with  the  cell  theory,  in  Chapter  IV,  we  have 
already  laid  stress  upon  the  fact  that  every  multicellular  animal 
or  plant  commences  its  individual  life  as  a  unicellular  egg  or 
ovum,  and  gradually  passes  by  slow  stages  of  cell-division  and 
differentiation  into  the  adult  condition.  There  are,  of  course, 
apparent  exceptions  to  this  rule  in  the  case  of  animals  and 
plants  which  reproduce  by  budding  or  by  some  analogous  process, 
where  the  bud  arises  from  a  group  of  cells  belonging  to  the 
parent,  but  even  in  these  cases  reproduction  by  means  of 
germ  cells  is  resorted  to  at  more  or  less  frequent  intervals  and 
the  budding  must  be  regarded  merely  as  an  additional  method  of 
multiplication  interpolated  in  the  life-cycle. 

The  individual  organism,  then,  does  not  come  into  existence 
fully  formed  in  all  its  perfection,  but  passes  through  a  longer  or 
shorter  process  of  development  to  the  adult  condition.  In  fact 
it  undergoes  a  process  of  individual  evolution  which  constitutes 
its  individual  life-history  or  ontogeny,  and  the  length  and 
complexity  of  this  process  are  proportional  to  the  complexity  of 
organization  of  the  adult.  Thus  the  complete  life-cycle  of  many 
of  the  lower  multicellular  animals  and  plants  is  passed  through 
in  the  course  of  a  few  months,  while  a  man  requires  twenty 
years  or  more  to  attain  his  full  development. 

Although,  for  the  sake  of  convenience,  the  ontogeny  of  any 
given  organism  may  be  divided  up  into  a  number  of  stages,  yet 
these  stages  cannot  in  reality  be  sharply  distinguished  from 
one  another.  Even  the  division  or  segmentation  of  the  ovum 
(Figs.  13  and  119),  whereby  the  organism  passes  from  the  uni- 
cellular to  the  multicellular  condition,  does  not  take  place 
suddenly  but  by  means  of  the  slow  and  complicated  process  of 
mitosis  or  karyokinesis  (Fig.  31)  ;  and  in  cases  where  there  is  an 
apparently  abrupt  change,  or  metamorphosis,  from  one  condition 


264        OUTLINES   OF   E  VOLUTION  ABY  BIOLOGY 

to  another,  as,  for  example,  at  a  later  stage,  from  the  chrysalis  to 
the  butterfly,  the  development  really  progresses  quite  slowly 
and  gradually  internally,  although  the  external  appearance  may 
for  a  long  time  remain  unaltered  and  give  the  impression  that 
it  is  completely  at  a  standstill. 

The  fact  that  the  young  organism  cannot  commence  its  life  at 
the  stage  reached  by  its  parents,  but  has  to  make  a  fresh  start 
from  the  beginning  and  go  through  a  whole  series  of  stages 


FIG.  117. — A.  Chick  Embryo  of  about  5^  days,  X  5.  B.  Eabbit  Embryo  of 
about  13  days,  X  5.  In  both  cases  the  foetal  membranes  and  yolk-sac 
have  been  removed.  (From  photographs.) 

before  reaching  the  adult  condition,  is  very  significant  from  the 
point  of  view  of  the  evolutionist.  Still  more  significant  is  the 
fact  that  different  organisms  all  commence  at  the  same  stage — 
as  unicellular  eggs — and  come  to  diverge  further  and  further 
from  one  another  in  structure  as  their  development  progresses. 
If  we  examine  a  number  of  series  of  embryos  belonging  to 
different  vertebrate  types,  no  matter  how  widely  the  adult 
animals  may  differ  from  one  another,  we  shall  find  that  as  we 
trace  their  life-histories  backwards  they  gradually  converge  until, 
while  still  at  a  relatively  advanced  stage  of  development,  the 
different  embryos  come  to  resemble  one  another  so  closely  that 


THE    RECAPITULATION   HYPOTHESIS  265 

in  many  cases  it  is  doubtful  if  even  an  experienced  embryologist 
could  distinguish  between  them.  This  great  generalization 
appears  to  have  been  first  reached  by  the  embryologist  von  Baer, 
who  in  1827  discovered  the  mammalian  ovum.  It  may  be 
illustrated  by  the  chick  and  rabbit  embryos  represented  in 
Fig.  117. 

Von  Baer's  generalization  contained  the  germ  of  what  is  now 
known  as  the  Recapitulation  Hypothesis,  or,  as  Haeckel  has 
termed  it,  the  Biogenetic  Law,  which  states  that  every  organism 
in  its  individual  life-history  recapitulates  the  various  stages 
through  which  its  ancestors  have  passed  in  the  course  of  their 
evolution.  In  other  words,  ontogeny,  or  the  life-history  of  the 
individual,  is  a  repetition  of  phylogeny,  or  the  ancestral  history 
of  the  race  to  which  the  individual  belongs. 

We  have  already  referred,  in  Chapter  IV  (Fig.  13),  to  some  of 
the  earlier  stages  in  the  development  of  that  primitive  fish- 
like  animal  Amphioxus  (Fig.  118).  Let  us  next  inquire  how 
these  stages  may  be  interpreted  in  accordance  with  the  recapitu- 
lation hypothesis.  The  unicellular  ovum  (Fig.  13,  I)  obviously 
represents  the  remote  protozoon  ancestors  which  were  common 
to  the  whole  animal  kingdom,  and  which  are  also  represented 
at  the  present  day  by  independent  unicellular  organisms  such 
as  the  Amoeba.  The  segmentation  of  the  ovum  into  primitive 
embryonic  cells  or  blastomeres  (Fig.  13,  II — VI)  represents  the 
transition  from  the  condition  of  the  simple  protozoon  to  that  of 
the  protozoon  colony,  in  which  the  individual  cells,  instead  of 
separating,  as  in  the  dividing  Amceba,  remain  together,  but  still 
without  undergoing  any  marked  differentiation  and  division  of 
labour.  The  arrangement  of  the  blastomeres  in  the  form  of  a 
hollow  sphere,  the  blastula  or  blastosphere  (Fig.  13,  VII),  with  a 
single  layer  of  cells  surrounding  a  central  cavity,  represents  the 
formation  of  such  a  protozoon  colony  as  we  see  in  the  existing 
Volvox  (Fig.  11)  or  Sphaerozoum  (Fig.  12).  The  process  of 
gastrulation,  whereby  the  single-layered  blastula  is  converted 
into  a  two-layered  gastrula  (Fig.  13,  VIII — X),  with  primitive 
digestive  cavity  (enteron)  and  primitive  mouth  (blastopore),  repre- 
sents the  transition  from  the  protozoon  colony  to  the  ccelenterate 
stage  of  evolution,  the  latter  being  still  represented  at  the  present 
day  by  such  forms  as  Hydra  (Fig.  57),  Obelia  (Fig.  60),  the  jelly- 
fish, the  corals  and  all  their  numerous  relations,  which  retain  in 
their  organization  all  the  essential  features  of  the  gastrula, 


266        OUTLINES   OF   E  VOLUTION  AKY  BIOLOGY 

though  generally  complicated  by  the  development  of  tentacles, 
skeleton,  &c.  The  development  of  coelomic  pouches  (Fig.  13, 
XI — XIII,  c.p.)  as  outgrowths  of  the  primitive  digestive  cavity, 
and  the  conversion  of  these  into  mesoblastic  somites  (Fig.  13, 
XV,  M.S.),  arranged  serially  or  metamerically  down  each  side  of 
the  body,  mark  the  transition  from  the  unsegmented  and 
ccelenterate  condition  to  the  metamerically  segmented1  and 
coelomate  condition.  The  cavities  of  the  coelomic  pouches  form 
the  coelom  or  body  cavity,  which  is  at  first  transversely  sub- 
divided into  compartments,  as  it  still  is  in  the  adult  earthworm, 
while  their  walls  form  the  third  germ-layer  or  mesoblast,  lying 
between  the  epiblast  which  covers  the  surface  of  the  body  and 


FIG.  118. — The  Lancelot  (A mpliioxuslavceolatus),  X  2.     (From  a  photograph.) 

the  hypoblast  which  lines  the  digestive  cavity.  Along  the  mid- 
dorsal  line  of  the  body  a  strip  of  epiblast  sinks  down  (Fig.  13, 
XI,  n.pl)  and  becomes  folded  into  the  form  of  a  tube  (Fig.  13, 
XII — XIV,  n.t.),  the  rudiment  of  the  central  nervous  system 
(brain  and  spinal  cord),  and  beneath  this  tube  a  long  strip  of 
hypoblast  becomes  nipped  off  from  the  roof  of  the  gut,  forming 
the  notochord  (Fig.  13,  XI — XIV,  not.)  or  axial  skeletal  rod— 
the  foundation  around  which  in  higher  types  the  vertebral 
column  is  built  up.  A  little  later  the  front  part  of  the  gut 
becomes  pierced  by  gill- slits  for  purposes  of  respiration  and 
the  primitive  chordate  condition  is  thus  fully  attained. 

Amphioxus  (Fig.  118)  does  not  progress  much  beyond  this 
stage  in  its  development.  It  never  acquires  any  limbs  but  swims 
about  by  lateral  undulations  of  the  body  caused  by  the  contraction 

1  Metameric  segmentation  has,  of  course,  nothing  to  do  with  the  segmentation  of 
the  ovum,  hut  is  the  term  applied  to  the  transverse  division  of  the  body  in  many 
animals  into  a  series  of  joints  or  segments  arranged  longitudinally,  each  of  which 
typically  repeats  the  essential  features  of  all  the  others. 


THE    RECAPITULATION   HYPOTHESIS  267 

of  the  lateral  sheets  of  muscle  derived  from  the  mesoblast.  No 
skull  and  no  vertebral  column  are  formed,  the  notochord 
remaining  throughout  life  as  almost  the  sole  representative  of  the 
internal  skeleton.  The  brain  does  not  become  distinctly 
differentiated  from  the  spinal  cord  and  no  paired  organs  of  sense 
are  developed,  nor  does  that  division  of  the  body  into  head  and 
trunk,  which  is  so  characteristic  of  typical  vertebrates,  take 
place.  Amphioxus  is  a  chordate  animal  but  it  is  not  a  true 
vertebrate.  It  probably,  however,  represents  fairly  closely  a 
stage  of  evolution  through  which  the  ancestors  of  the  vertebrates 
have  passed,  though  it  becomes  somewhat  modified  by  secondary 
features  in  the  later  stages  of  its  development. 

It  is  easy  enough  to  interpret  the  earlier  development  of  Amphi- 
oxus in  terms  of  the  recapitulation  hypothesis  and  to  recognize 
in  the  ontogeny  certain  stages  of  the  phylogenetic  history.  The 
ontogenetic  record,  however,  is  by  no  means  always  so  readily 
decipherable.  It  is  generally  more  or  less  obscured  by  secondary 
features  superposed  upon  it,  much  as  an  ancient  picture  may 
be  concealed  by  another  painted  over  it.  These  secondary 
features  are  characters  which  are  developed  in  relation  to  the 
requirements  of  the  embryo  itself  at  its  various  stages,  for  it  is 
not  only  the  adult  organism  which  becomes  modified  in  structure 
in  the  course  of  evolution  but  all  the  stages  of  its  life-history 
are  likewise  subject  to  adaptation. 

In  the  first  place  nourishment  has  to  be  provided  for  the 
developing  embryo,  and  this  necessity  is  present  from  the  very 
first  division  of  the  fertilized  ovum.  The  blastomeres  into 
which  it  divides  cannot  go  about  and  seek  their  own  food  supplies 
like  so  many  Amoebae,  they  have  sacrificed  the  power  of  leading 
independent  lives  for  the  sake  of  remaining  together  and  building 
up  a  more  or  less  complex  multicellular  body ;  but  none  the  less 
they  require  feeding.  Hence  we  find  that  all  eggs  contain  a 
larger  or  smaller  quantity  of  nutrient  material  stored  up  in 
the  cytoplasm.  This  usually  takes  the  form  of  definite  granules 
or  particles  of  food-yolk  (deutoplasm),  the  amount  of  which 
varies  immensely  in  different  cases.  If  there  is  very  little  the 
egg  is  said  to  be  microlecithal  as  in  Amphioxus ;  if  there  is 
much  it  is  said  to  be  megalecithal,  as  in  the  frog  and  still  more 
so  in  the  bird,  and  the  size  of  the  egg,  as  we  have  already  had 
occasion  to  point  out  in  Chapter  X.  depends  almost  entirely  upon 
the  amount  of  food-yolk  which  it  contains. 


268        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

». 

The  possession  of  food-yolk  is,  of  course,  of  immense  impdlt- 
ance  to  the  embryo,  for  the  stage  which  it  is  able  to  reach  in  ifli 
development  before  it  has  to  begin  to  provide  for  itself  will 
depend  upon  the  amount  of  nutrient  material  which  it  has  at  its 
disposal.  The  presence  of  this  food-yolk,  however,  must  also 
act  as  a  mechanical  hindrance  to  the  development,  just  as  the 
supply  of  provisions  which  he  carries  with  him  must  impede  the 
progress  of  a  traveller,  though  enabling  him  ultimately  to 
accomplish  a  longer  journey. 

In'  Amphioxus  the  egg  contains  hardly  any  food-yolk  and  the 
process  of  segmentation  and  the  formation  of  the  blastula  and 
gastrula  take  place  in  an  almost  perfectly  typical  manner. 
There  is  here  practically  no  hindrance  to  development  and  no 
obliteration  of  the  ontogenetic  record.  There  is,  however,  just 
sufficient  nutrient  material  to  cause  some  of  the  cells — those 
which  will  invaginate  to  form  the  hypoblast  of  the  gastrula — to 
be  slightly  larger  than  the  remainder,  which  are  destined  to  form 
the  epiblast  (Fig.  13,  VII,  VIII). 

Let  us  compare  with  this  the  corresponding  stages  in  the 
development  of  the  frog  (Fig.  119).  Here  the  egg  is,  as  we  have 
already  indicated,  much  larger,  owing  to  the  greater  quantity  of 
food-yolk  which  it  contains.  This  is  chiefly  collected  in  the 
lower  half  of  the  ovum,  while  the  upper  half  is  deeply  pigmented 
and  thus  readily  distinguished.  Segmentation  begins  as  in , 
Amphioxus  by  the  formation  of  two  vertical  clefts,  or  cell-divisions, 
one  after  the  other  and  at  right  angles  to  one  another,  whereby 
the  four-celled  stage  is  reached  (Fig.  119,  I — III).  Each  cleft 
begins  at  the  pigmented  pole  and  throughout  the  whole  of  the 
segmentation  the  yolk-laden  half  of  the  egg  lags  behind  the 
pigmented  half  because  of  the  mechanical  hindrance  which  the 
food-yolk  opposes  to  the  process  of  cell-division.  The  first 
horizontal  cleft  (Fig.  119,  IV)  divides  each  of  the  four  blasto- 
meres  into  two  daughter  cells  of  very  unequal  size,  an  upper 
pigmented  cell  with  little  food-yolk  and  a  much  larger,  lower, 
yolk-laden  cell  with  little  pigment.  After  this,  segmentation 
continues  to  progress  more  rapidly  in  the  upper  than  in  the 
lower  half  of  the  egg  (Fig.  119,  V),  until  we  reach  the  blastula 
stage  (Fig.  119,  VI),  where  we  find  the  large  yolk-laden  cells 
forming  the  lower  half  of  the  wall  of  the  hollow  sphere  and  pro- 
jecting into  and  partly  obliterating  its  cavity,  while  the  small 
pigmented  cells  form  the  upper  half. 


EAKLY  DEVELOPMENT  OF  THE  FROG 


269 


«he  process  of  gastrulation  is  profoundly  modified  by  the 
ience  of  the  food-yolk,  and  instead  of  a  simple  invagination 
of  the  lower  half  of  the  blastula  wall  to  form  the  hypoblast,  we 
find  the  small  (epiblast)  cells,  now  arranged  in  several  layers, 
spreading  themselves  over  the  yolk-containing  cells  until  they 
completely  enclose  the  latter  except  for  a  small  circular  area 


Uc. 


FIG.  119.— Early  Stages  in  the  Development  of  the  Frog  (partly  from 
Ziegler's  models  and  partly  adapted  from  Marshall.) 

J,  the  fertilized  ovum  ;  JJ — V,  segmentation  of  the  ovum  ;  VI,  blastula  ;  VII,  modified 
blastula,  with  wall  composed  of  more  than  one  layer  of  cells ;  VIII,  commencement 
of  gastrulation;  IX,  the  modified  gastrula  stage  (VI — IX  in  vertical  section). 

blc.,  blastocoel  or  segmentation  cavity;  bp.,  blastopore ;  <sw£.,'enteron;  ep.,  epiblast; 
hyp.,  hypoblast ;  u.l.bp.,  upper  lip  of  blastopore;  yk.c.,  yolk-containing  cells.  - 

which  corresponds  to  the  blastopore  or  aperture  of  invagination 
in  Amphioxus  blocked  up  by  yolk  cells  (Fig.  119,  VIII,  IX). 
Indeed,  practically  the  whole  of  the  primitive  digestive  cavity  or 
enteron  in  the  frog's  gastrula  may  be  regarded  as  being  filled 
with  yolk  cells,  from  which  hypoblast  will  be  derived,  and  it 
only  gradually  opens  out  later  on  in  development  as  the  yolk  is 
used  up.  Hence  the  coelenterate  stage  of  the  ancestral  history 


270        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

(Fig.  119,  IX)  is  very  much  disguised  in  the  ontogeny,  simply 
owing  to  the  presence  of  a  large  quantity  of  nutrient  material 
in  the  egg. 

In  the  development  of  the  bird's  egg  (and  also  in  that  of  reptiles, 
which  is  closely  similar)  it  is  still  more  difficult  to  recognize  the 
process  of  gastrulation.  The  quantity  of  yolk  (Fig.  70)  is  so 
enormous  that  it  entirely  ^prevents  cell-division  from  taking 
place  in  the  greater  part  of  the  egg,  and  segmentation  is  con- 
sequently confined  to  a  small  yolk -free  area  at  one  pole  of  the 
spherical  mass,  where  it  gives  rise  to  a  layer  of  cells  known  as  the 
blastoderm.  This  blastoderm  gradually  spreads  over  the  yolk  and 
from  it  the  embryo  is  formed  by  a  kind  of  pinching-off  process, 
the  yolk  remaining  enclosed  in  a  bag — the  yolk-sac — attached  to 
the  lower  surface  of  the  embryo,  until  it  is  all  absorbed  by  means 
of  a  special  set  of  blood-vessels  developed  in  the  yolk-sac,  and 
used  up  in  the  nourishment  of  the  growing  embryo.  The 
embryo  itself  meanwhile  becomes  provided  with  special  organs 
for  its  protection  and  respiration,  the  so-called  foetal  membranes. 
Of  these  the  amnion,  formed  by  outgrowth  of  the  embryonic 
body-wall,  forms  a  sac  filled  with  fluid,  which  surrounds  the 
embryo  at  a  distance  and  leaves  it  free  room  for  growth,  while 
the  allantois,  formed  by  outgrowth  of  the  embryonic  gut- wall, 
forms  a  highly  vascular  organ  which  comes  into  relation  with 
the  extremely  porous  shell  and  thus  provides  for  the  necessary 
gaseous  interchange. 

The  yolk-sac,  the  amnion  and  the  allantois  are  all  organs 
which  have  been  developed  in  accordance  with  the  special 
requirements  of  the  embryo,  and  which  must  therefore  be  left 
out  of  account  in  reconstructing  the  direct  ontogenetic  record. 

In  the  dogfish  also  (Fig.  120)  we  find  an  excellent  illustration 
of  the  formation  of  a  special  yolk-sac  to  contain  the  enormous 
supply  of  food  material  for  the  developing  embryo,  but  in  this 
case  neither  amnion  nor  allantois  is  developed. 

The  Mammalia,  on  the  other  hand,  have  adopted  an  improved 
method  of  nourishing  their  young.  The  developing  embryos  are 
retained  within  the  body  of  the  parent  and  obtain  their  food 
supply  from  the  blood  of  the  latter  by  means  of  a  special  organ 
known  as  the  placenta,  which  is  developed  partly  from  the 
allantois  of  the  embryo  or  "  foetus,"  and  by  which  the  latter  is 
attached  to  the  wall  of  the  uterus.  This  method  seems  to  have 
proved  much  more  satisfactory  than  the  provisioning  of  the  egg 


INFLUENCE    OF   FOOD  YOLK  271 

with  food-yolk,  and  we  accordingly  find  that  the  mammalian 
ovum  (Fig.  71)  has  become  reduced  in  size  again  until  it  is 
no  larger  thin  that  of  Amphioxus,  i.e.  about  aio^h  incn  i*1 
diameter.  The  early  stages  of  segmentation  have  also  gone 
back  to  the  primitive  type,  but,  strange  to  say,  a  yolk-sac  is  still 
formed  although  there  is  no  yolk  for  it  to  contain — affording  an 
admirable  example  of  a  vestigial  embryonic-organ.  Here  one 
record  has  been  superposed  upon  another  in  the  ontogeny  and 
we  have  a  recapitulation  of  an  embryonic  stage  of  the  reptilian 
ancestors  of  the  mammals.  After  birth,  of  course,  the  young 


FIG.  120. -Embryo  of  a  Dogfish,  with  Yolk-Sac  (Y.S.)  attached,  X  2.     (From 

a  photograph.) 

mammal  is  nourished  by  the  secretion  of  milk,  and  is  thereby 
enabled  to  reach  a  very  advanced  stage  of  development  before  it 
has  to  begin  to  find  jts  own  food.  The  chick,  owing  to  the  very 
large  amount  of  food-yolk  with  which  the  egg  is  supplied,  is  also 
able  to  reach  a  highly  developed  condition  before  it  hatches  and 
begins  to  look  after  itself. 

With  animals  whose  eggs  contain  insufficient  food-yolk,  and 
whicli  are  not  provided  with  any  other  special  means  of 
nourishing  the  developing  young,  the  case  is  very  different.  They 
are  obliged  to  hatch  and  begin  life  on  their  own  account  while 
still  a  long  way  from  the  adult  condition — when  only  a  small 
part  of  their  ontogenetic  journey  has  been  accomplished. 
Amphioxus,  for  example,  hatches  as  a  free-swimming  ciliated 
embryo  shortly  after  the  gastrula  stage  has  been  passed  through, 
and  completes*  its  development  at  the  expense  of  food  supplies 
which  it  obtains  for  itself. 


272        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Embryos  which,  having  hatched  at  a  comparatively  early  stage 
of  their  development,  lead  independent,  self-supporting  lives,  are 
commonly  spoken  of  as  larvae,  and  amongst  such  larvae  we  again 
find  abundant  instances  oT  special  larval  organs  developed  in 
relation  to  their  special  requirements. 

The  tadpole  of  the  frog  (Fig.  121,  4—11)  affords  an  excellent 
example  of  a  free-living  larva.  It  has  certain  special  organs, 
such  as  its  horny  jaws  and  the  suckers  on  the  under  surface  of 


FIG.  121. — Stages  in  the  Life  History  of  the  Frog.    (From  Brehm's  "  Thier- 

leben.") 

1,  freshly  laid  eggs;  2,  eggs  slightly  magnified,  adhering  together  by  their  gelatinous 
envelopes  to  form  ''  spawn  "  ;  3,  young  tadpole  still  enclosed  in  the  jelly  ;  4,  newly 
hatched  tadpoles  adhering  by  their  suckers  to  a  blade  of  grass ;  5,  6,  tadpoles  with 
external  gills ;  7,  8,  tadpoles  with  the  gills  concealed  by  the  operculum ;  9,  10,  tad- 
poles with  conspicuous  hind  legs  but  with  the  front  legs  still  concealed  beneath  the 
operculum;  11,  tadpole  during  metamorphosis,  with  front  legs  protruded ;  12,  young 
frog  with  tail  not  completely  absorbed. 

(For  microscopic  details  of  the  early  stages  see  Fig.  119.) 

the  head,  which  are  developed  in  relation  to  its  larval  Jife,  but 
there  can  be  no  question  about  the  phylogenetic  significance 
of  the  tadpole  stage  as  a  whole.  In  every  essential  feature  of  its 
organization  the  tadpole,  before  its  legs  develop,  is  a  fish.  It 
swims  like  a -fish  by  means  of  its  well  developed  tail,  it  breathes 
like  a  fish  by  means  of  its  gills  and  gill-slits,  and  its  internal 
structure  is  that  of  a  fish.  There  can  be  no  doubt  that  it 
represents  a  fish-like  stage  in  the  evolution  of  the  frog. 

In  the  tadpole,  owing  to  its  free-living,  aquatic  mode  of  life, 


ABBKEVIATION   OF   ONTOGENY 


'273 


du 


the  characteristic  piscine  organs  are  fully  functional,  but  even  in 
the  embryos  of  higher  air-breathing  vertebrates — reptiles,  birds 
and  mammals — while  enclosed  within  the  egg-shell  or  within  the 
womb  of  the  parent,  we  can  still  recognize  clear  traces  of  a 
similar  fish-like  stage  in  their  evolution,  for  gill-slits  are  present 
in  all  (Fig.  122,  g.s.)  and  the 
arrangement  of  the  principal  veins 
and  arteries  is  unmistakably  fish- 
like. 

Difficulties  in  the  interpretation  cf 
the  ontogenetic  record  arise  not  only 
from  the  addition  of  new  features 
in  the  form  of  embryonic  or  larval 
organs  but  also  from  the  abbreviation 
of  the  different  stages  through  which 
the  organism  passes.  The  develop- 
ment of  any  of  the  higher  organisms 
must  be  looked  upon  as  a  kind  of 
compromise,  due  to  the  necessity  of 
attaining  as  perfect  a  condition  as 
possible  in  as  short  a  time  as  pos- 
sible, for  the  sooner  the  adult  condi- 
tion is  reached  the  sooner  will  the 
organism  be  able  to  reproduce  its 
kind,  and  hence  any  abbreviation  of 
the  earlier  stages  of  development  will 
be  advantageous  to  the  species  and 
will  be  favoured  by  natural  selection. 
For  this  reason  the  fish-like  stage 
in  the  development  of  reptiles,  birds 
and  mammals  is  passed  through  very 
rapidly  and  the  fish-like  structure  is 
no  longer  fully  developed.  Although  gill-slits  are  present  the 
gills  themselves  are  not,  and  other  piscine  characters  are  only 
represented  by  transient  vestiges. 

Turning  now  to  some  of  the  lower  groups  of  the  animal  king- 
dom, we  may  briefly  notice  a  beautiful  illustration  of  the  law  of 
recapitulation  afforded  by  the  life-history  of  the  common  feather- 
star,  Antedon  (Fig.  123).  This  animal,  found  abundantly  in 
comparatively  sjiallow  water  off  the  British  coast,  belongs  to  the 
great  phylum  Echinodermata,  which  also  includes  the  star-fish, 


FIG.  122.— Chick  Embryo  of 
about  3  Days,  with  the 
Foetal  Membranes  and 
Yolk -Sac  removed.  (Pho- 
tographed from  one  of 
Ziegler's  models.) 

au.,  auditory  organ  ;  e.,  eye  ;  g.s.,  gill 
slits;  n.,  heart;  m.s.,  meso- 
blastic  somites ;  n.,  nasal  organ. 


274      OUTLINES  OF   EVOLUTIONARY   BIOLOGY 

sea- urchins  and  sea-cucumbers.  The  subdivision  to  which  ifc 
belongs — the  Crinoidea — is  characteristically  a  deep-water  group. 
It  is  also  a  group  of  great  antiquity,  the  remains  of  crinoids — 
such  as  the  well-known  "  Encrinites  "  of  the  Carboniferous  period 
—being  abundant  in  certain  Palaeozoic  formations. 

Both  the  Palaeozoic  crinoids  and  the  surviving  deep-sea  mem- 
bers of  the  group,  such  as  Pentacrinus  (Fig.  125),  are  stalked 
forms,  the  "  calyx,"  with  its  radiating  arms,  being  attached  to 


FIG.  123.  FIG.  124. 

FIG.  123.— The  Feather  Star  (Antedon  lifida},  nat.  size. 
FIG.  124. — Pentacrinoid  Stage  in  the  development  of  Antedon,  X  14. 
dr.  cirri ;  st.  stalk. 

the  end  of  a  long,  jointed,  calcareous  stem,  the  lower  extremity  of 
which  is  permanently  fixed  to  the  sea-bottom.  Antedon,  and  the 
other  shallow-water  crinoids  of  the  present  day,  on  the  other 
hand  (Fig.  123),  have  no  stalks  but  in  place  thereof  a  number  of 
slender  "  cirri "  whereby  they  temporarily  attach  themselves  to 
seaweed  or  other  objects. 

Now  in  the  course  of  its  development  from  the  egg  the  feather- 
star  passes  first  through  a  free-swimming  larval  stage  and  then 
through  a  fixed  stalked  stage  (Fig.  124),  known,  from  its  resem- 
blance to  the  deep-sea  genus  Pentacrinus,  as  the  pentacrinoid 


LARVAL  ORGANS 


275 


FIG.  125. — A  Deep-Sea  Crinoid,  Penta- 
crinus  (Isocrinus)  decorus,  about  iiat. 
size. 


Later  on  the  young  animal  develops  cirri  and  falls  off 
the  end  of 
the     stalk, 
assuming 

once  more 
an  active 
modeoflife. 
Here  we 
recognize 
very  clearly 
in  the  pen- 
tacrinoid 
stage  of  the 
ontogeny 
a  repeti- 
tion of  the 
stalked 
condition 

through  which  the  ancestors  of  the  feather-star 
must  have  passed.  / 

We  also  find  amongst  invertebrate  animals 
abundant  examples  of  special  larval  organs. 
The  body  of  a  caterpillar,  for  instance,  is  built 
up  very  largely  of  such  structures,  and  it  has 
to  undergo  an  almost  complete  reconstruction 
within  the  protective  envelope  of  the  chrysalis 
before  it  reaches  the  adult  condition  and  emerges 
as  a  perfect  insect.  It  is  therefore  extremely 
difficult  to  interpret  the  caterpillar  stage  in 
terms  of  the  ancestral  history. 

The  ophiuroids  or  brittle  stars  (Fig.  126),  so 
commonly  met  with  between  tide-marks,  have 
remarkable,  free-swimming  "  Pluteus  "  larvae 
(Fig.  127),  totally  different  in  appearance  from 
the  adult.  These  larvae  occur  in  immense 
numbers  in  the  surface  waters  of  the  ocean. 
They  are  provided  with  long  slender  arms 
supported  on  calcareous  rods,  which  probably 
serve,  in  part  at  any  rate,  to  protect  the  very 
delicate  body  from  the  attacks  of  other  small  animals.  Before 
attaining  the  adult  condition  the  larva  undergoes '  a  complex 


276       OUTLINES   OF  EVOLtJTIONAKY  BIOLOGY 

metamorphosis,  during  which  the  long  arms  are  completely  lost 
and  the  form  of  the  body  entirely  changed.  The  echinoids  or 
sea-urchins  pass  through  a  very  similar  larval  stage. 

The  free-swimming  "  Zoaea  "  larva  of  the  crab  (Fig.  128)  is  also 
provided  with  conspicuous  defensive  spines  which  are  lost  in  the 


FIG.  12G. 


Fj4.  127. 

FIG.  126.— A  Brittle  Star  (Ophiura  aliaris),  X  £. 
FIG.  127.— The  Plutetfs  Larva  of  a  Brittle  Star,  : 


x  62. 


adult  condition  (Fig.  102)  and  which  have  doubtless  been  developed 
in  relation  to  the  special  conditions  of  the  larval  life. 

In  many  cases  the  development  of  larval  organs  causes  the 
young  animal  to  differ  so  widely  from  the  adult  that  the  relation- 
ship of  the  two  for  a  long  time  remained  unsuspected,  and  hence 
the  special  names  which  many  larvae  have  received.  In  other 
cases,  however,  it  is  the  adult  animal  which  has  become  so  pro- 
foundly modified  in  relation  to  special  conditions  of  life  as  to 


INTEKPBETATION   OF   ONTOGENY 


277 


obscure  its  genetic  affinities,  which  may  still  be  clearly  indicated 
by  embryonic  or  larval  stages. 

The  common  Ascidian,  for  example  (Fig.  129),  is  a  sac-shaped 
organism,  permanently  attached  to  rock  or  seaweed  in  the  adult 
condition,  which  by  the  uninitiated  would  hardly  be  taken  for  an 
animal  at  all.     Before  their  life-history  was  known  the  Ascidians 
were  placed  by  zoologists  amongst  the  "  Molluscoida,"  a  group  of 
invertebrates  supposed  to  resemble  the  mollusca  or  shell-fish.    It 
was  only  the  discovery  that 
the    typical    Ascidians    pass 
through    an    active    tadpole 
stage   in   their   development 
(Fig.  130),  with  a  muscular 
tail,  a  notochord  and  a  central 
nervous  system  all  formed  as 
in   vertebrates,  that  demon- 
strated   their    true    position 
as    degenerate    members    of 
the   great  phylum  Chordata 
(=  Vertebrata  in  the  widest 
sense  of  the  term),  the  tad- 
pole stage  being,  of  course, 
a  recapitulation  of  the  ances- 
tral, fish-like,  chordate  con- 
dition. 

It  will  be  readily  under- 
stood from  the  joregoing 
illustrations  jhat^  in^endea- 
"vouring  to  Reconstruct  the- 
ancestral  history  of  an  organ- 
ism from  the  stages  which  it 
passes  through  in  its  indivi- 
dual development,  we  have  to  distinguish  very  carefully  between 
two  sets  of  characters.  In  the  first  place  there  are  characters  which 
are  due  to  inheritance  from  the  adult  condition  of  very  remote 
ancestors  and  which  really  indicate  stages  in  what  we  may  call 
the  direct  line  of  evolution.  These  are  termed  palingenetic  or 
ancient  characters.  Such,  for  example,  are  the  essential  features 
of  the  gastrula  stage,  which  occurs  so  generally  and  which  indi- 
cates a  remote*  coelenterate  ancestor  (the  Gastrsea  of  Professor 
Haeckel)  for  at  any  rate  the  great  majority  of  multicellular  animals. 


FIG.  128.— The  Zoom  Larva  of  a 
Crab  (Portunus),  X  30.  (From  a 
photograph.) 


278        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Such  also  are  the  essential  characters  of  the  tadpole  stage  of  the 
frog  and  of  the  ascidians,  and  the  vestigial  gill-slits  of  higher 
vertebrates,  which  indicate  descent  from  some  gill-breathing,  fish- 
like  form. 

In  the  second  place  there   are   characters  which  have  been 
acquired  secondarily  by  the  embryo  or  larva  in  relation  to  its 


ejc.ap. 


\ 


FIG.  129.— A  Simple  Ascidian  (dona 
intestinalis*),  X  ^. 

a.p.j  processes  for  attachment ;  in.ap., 
inhalant  aperture;  ex.ap.,  exhalant 
aperture. 


FIG.  130.  —  Tadpole  Larva  of  a 
Compound  Ascidian  (Diplosoma 
crystallinum\,  X  35.  (From  a 
photograph.) 

p.,  papillae  for  attachment ;  t.,  tail. 


special  embryonic  or  larval  requirements.  These  are  termed 
caenogenetic  or  modern  characters,  and  it  is  these  which  tend 
more  or  less  to  obscure  the  ontogenetic  record.  Such  are  the 
presence  of  food-yolk,  the  foetal  membranes  of  air-breathing 
vertebrates  and  the  various  special  larval  organs  to  which  we 
have  above  referred.  It  is  difficult,  however,  to  draw  a  hard  and 
fast  line  between  palingenetic  and  caenogenetic  characters,  and 
we  must  always  remember  that  embryonic  stages,  as  well  as  the 


PHYLOGENY  AND  ONTOGENY 


279 


Stages    in    OnCogeny  • 


adult,  undergo  modification  during  evolution,  and  that  some 
caenogenetic  characters  are  very  much  more  ancient  than  others. 
The  relation  which  exists  between  the  phylogenetic  and  onto- 
genetic stages  in  the  evolution  of  an  organism  are  very  diagram - 
matically  expressed  in  Fig.  131,  which  is  drawn  with  special 
reference  to  the  familiar  case  of  the  frog.  The  vertical  line  is 
supposed  to  represent  the  ancestral  history,  some  of  the  principal 
stages  of  which  are  indicated. 
The  horizontal  line  at  the  top 
represents  the  individual  life- 
history,  with  the  stages  indi- 
cated which  represent  those 
selected  in  the  ancestral  his- 
tory. The  ovurn  represents 
the  protozoon  ancestor;  the 
blastula  the  hollow,  spherical 
protozoon  colony;  the  gastrula 
the  ccelenterate ;  the  meta- 
merically  segmented  embryo 
the  segmented  worm,  and  the 
tadpole  the  fish-like  stage. 
The  oblique  lines  represent 
the  lines  of  evolution  of  the 
different  ontogenetic  stages 
themselves,  in  the  course  of 
which  caenogenetic  characters, 
such  as  the  acquisition  of 
food-yolk  by  the  ovum  and  of 
suckers  and  horny  jaws  by 
the  tadpole,  have  arisen.  The 
shorter  horizontal  lines  repre- 


Protozoon 


FIG.  131.  —  Diagram  illustrating  the 
Eelation  between  Ontogeny  and 
Phylogeny. 


sent  the  ontogeny  at  the  different  ancestral  stages,  and  their 
increase  in  length  from  below  upwards  is  obviously  due  to  the 
successive  addition  of  new  stages  to  the  ontogenetic  record,  while 
the  shortness  of  the  horizontal  lines  as  compared  with  the  corre- 
sponding portions  of  the  vertical  line  may  be  taken  to  indicate, 
though  very  imperfectly,  the  abbreviation  of  the  ontogeny. 

.In  the  vegetable  kingdom  we  find  illustrations  of  the  law  of 
recapitulation  and  of  the  complication  of  the  ontogenetic  record 
by  the  development  of  caenogenetic  characters,  quite  as  striking 
as  any  which  we  find  amongst  animals.  Take,  for  example,  the 


280        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


life-history  of  some  of  the  Australian  acacias,  or  wattles.  These 
are  .leguminous  plants  and  many  of  them  have  the  pinnately 
subdivided  leaves  so  characteristic  of  the  order.  Others,  however, 
have  the  pinnate  leaves  in  the  adult  entirely  replaced  by  narrow 
"phyllodes,"  which  closely  resemble  simple  undivided  leaves 
like  those  of  some  of  the  eucalypts,  but  are  really  only  flattened 
leaf-stalks  or  petioles.  The  development  of  such  phyllodes  is  no 
doubt  to  be  regarded  as  an  adaptation  to  the  heat  and  drought  of 

the  Australian  climate, 
which  delicately  sub- 
divided leaves  of  the 
ordinary  leguminous 
type  are  ill-suited  to 
withstand,  and  the  re- 
semblance of  the  phyl- 
lodes to  true  foliage 
leaves  of  the  undivided 
type — and  especially  to 
those  of  the  eucalypis 
— affords  a  very  good 
illustration  of  the 
phenomenon  of  conver- 
gence.  If,  now,  we 
examine  the  seedlings 
of  a  phyllode-bearing 
acacia  (Fig.  132)  we 
shall  find  that  imme- 
diately after  the  coty- 
ledons or  seed  -  leaves 
they  bear  typical  pin- 
nate leaves,  and  then 
leaves  of  intermediate  form,  thus  clearly  recapitulating  the 
ancestral  condition  from  which  the  phyllode-bearing  forms  were 
derived.  A  very  similar  state  of  things  occurs  in  the  common 
European  gorse  or  furze,  another  leguminous  plant,  in  which 


FIG.  132.  —  Seedling  of  a  phyllode-bearing 
Acacia  (Acacia  pycnahtha].  (From  Stras- 
burger.)  (The  cotyledons  have  already 
been  shed.) 


the  leaves  of  the  adult  have  become  mo 
while  those  of  the   seedling   still   retai 
condition,  somewhat  resembling  those  o: 
The  cotyledons   or   true   seed-leaves 
the  other  hand,  afford  excellent  examples  of 


'form  spines, 
al  ternate 


g  plants,  on 
genetic  characters 


developed  in  relation  to  the  special  requirements  of  the  embryo. 


RECAPITULATION   AND   HEREDITY 


281 


They  are  very  commonly  modified  to  form  receptacles  for  a 
store  of  food  material  for  the  young  plant  in  the  form  of 
starch,  as  in  the  peas  and  beans  (compare  Fig.  56),  and  may 
never  even  leave  the  seed-coat  or  emerge  above  the  ground. 
In  such  cases  they  develop  no  chlorophyll  and  differ  absolutely 
from  ordinary  foliage  leaves,  without,  of  course,  indicating  any 
ancestral  stage  through  which  the  latter  have  passed.  Even 
when  they  emerge  above  the  ground,  as  in  the  beech  (Fig.  133), 
and  become  green,  their  form  is  usually  widely  different  from 
that  of  the  foliage  leaves  and 
appears  to  have  been  developed 
largely  in  adaptation  to  the  neces- 
sity for  close  package  within  the 
narrow  compass  of  the  seed-coat, 
the  shape  of  the  cotyledons  being 
governed,  as  Lord  Avebury  has 
pointed  out,  by  that  of  the  seed 
within  which,  in  a  dormant  condi- 
tion, they  pass  most  of  their  lives. 
In  short,  the  embryological 
investigation  of  both  animal  and 
vegetable  organisms  leaves  no 
doubt  as  to  the  general  truth 
of  the  recapitulation  hypothesis, 
and  must  convince  any  unbiassed 
observer  that,  however  much  modified  it  may  be  by  abbrevia- 
tion and  by  the  superposition  of  secondary  features,  the  life-history 
of  the  individual  is  essentially  a  condensed  epitome  of  the 
ancestral  history  of  the  race.  The  law  of  recapitulation,  indeed, 
may  be  regarded  simply  as  a  logical  extension  of  the  law  of 
heredity,  for  every  organism  tends  to  inherit  the  characters  not 
only  of  its  immediate"  prog^nrtoTS-imt  of  air  its  ancestors,  and 
these  characters  appear  in  the  individual  life-history  in  the  same 
order  as  that  in  which  they  first  appeared  in  the  ancestral 
history — in  other  words,  ontogeny  is  a  repetition  of  phylogeny 
and  can  onlv  be  ex^kined  in  terms  of  organic  evolution. 


FIG.  133.  —  Seedling  of  Beech, 
showing  the  Cotyledons. 
(From  Lubbock.) 


CHAPTEK  XIX 

The  stratified  rocks — Geological  periods — The  age  of  the  habitable  earth — 
The  geological  record — The  succession  of  the  great  vertebrate  groups. 

THE  materials  of  which  the  solid  crust  of  the  earth  is  made  up 
are,  as  it  is  perhaps  hardly  necessary  to  point  out,  extremely 
varied,  but  whatever  may  be  their  physical  or  chemical  characters 
they  are  all  spoken  of  by  geologists  as  rocks.  In  accordance 
with  their  mode  of  formation  these  rocks  may  be  divided  into  two 
chief  categories,  first,  those  which,  like  granite  and  basalt,  have 
been  formed  directly  by  cooling  of  molten  matter,  of  which  the 
earth  was  perhaps  at  one  time  entirely  composed,  and,  second, 
those  which,  like  sandstone,  shale,  limestone  and  coal,  have  been* 
formed  by  accumulation  of  detritus  derived  from  pre-existing 
rocks  or  from  the  remains  of  dead  organisms.  The  first  are 
commonly  termed  igneous  and  the  second  sedimentary,  the 
latter  deriving  their  name  from  the  fact  that  they  have  been 
deposited  under  water.  Owing  to  the  manner  in  which  •  they 
have  been  laid  down,  usually  in  the  form  of  gravel,  sand,  mud  or 
ooze,  which  have  subsequently  become  hardened  and  consolidated 
by  the  pressure  of  more  recent  deposits,  the  sedimentary  rocks 
form  a  series  of  layers  or  strata  of  varying  thickness  superposed 
one  upon  another.  The  stratification  must  at  first  have  been 
horizontal,  but,  owing  to  the  irregular  shrinkage  of  the  earth's 
crust  and  to  the  action  of  volcanic  upheavals,  it  has  usually  been 
more  or  less  disturbed,  so  that  the  planes  of  bedding  have  been 
tilted  at  various  angles  and  frequently  exhibit  a  high  degree  of 
curvature  or  even  contortion.  Moreover,  rocks  which  were 
originally  laid  down  on  the  bed  of  the  sea  have  in  many  cases 
come  to  form  part  of  the  dry  land  and  even  to  be  uplifted  far  above 
sea-level  in  the  form  of  mountain  ranges. 

The  formation  of  sedimentary  rocks  must  have  been  going  on 
ever  since  the  earth  became  cool  enough  to  admit  of  the  con- 
densation of  aqueous  vapour  into  water,  and  it  is  still  going  on 
in  the  same  way  at  the  present  day.  Every  river  is  continually 


THE    STRATIFIED   ROCKS  283 

carrying  down  to  the  sea  immense  quantities  of  sand  and  mud, 
which  will  ultimately  be  deposited  in  the  form  of  new  strata ; 
every  cliff  by  the  sea-shore  is  slowly  or  rapidly  weathering  away 
under  the  influence  of  atmospheric  agencies — rain,  frost,  tides 
and  wind  ;  and  far  out,  beyond  the  reach  of  the  detritus  derived 
from  the  land,  the  sea-bottom  is  being  covered  by  the  slow 
accumulation  of  ooze  or  mud  derived  almost  entirely  from  the 
remains  of  marine  organisms,  while  the  activities  of  corals  and 
other  reef-building  animals  in  their  turn  make  no  small  contribu- 
tion to  the  grand  total. 

The  sedimentary  rocks  naturally  contain  the  remains  of  vast 
numbers  of  organisms  which  flourished  on  the  earth  at  more  or 
less  remote  periods  of  its  history,  and  the  study  of  these  fossils 
may  justly  be  expected  to  yield  results  of  the  greatest  importance 
from  the  point  of  view  of  the  theory  of  organic  evolution.  We 
have  here,  in  fact,  the  only  really  direct  evidence  of  the  course 
which  the  evolution  of  the  animal  and  vegetable  kingdoms  has 
actually  taken,  and  this  evidence  constitutes  what  is  commonly 
known  as  the  geological  record. 

The  relative  ages  of  the  different  sedimentary  rocks  and  of 
the  different  geological  formations  which  they  build  up  are  of 
course  determined  primarily  by  their  position  with  regard  to  one 
another,  each  layer  or  bed  being  necessarily  of  more  recent  date 
than  the  one  below  it.1  The  series  of  strata  met  with  in  any 
particular  locality  may,  however,  be  very  different  from  lhat 
which  occurs  in  other  localities,  for  while  mud  may  be  accumu- 
lating in  one  place,  sandstone  may  be  forming  in  another  and 
limestone  in  a  third,  and  over  other  areas,  which  lie  above 
sea-level,  rock  formation  will  be  replaced  by  destruction  or 
denudation.  Nevertheless,  geologists  have  found  it  possible  to 
compare  the  series  of  stratified  rocks  found  in  different  parts  of 
the  world  one  with  another,  and,  by  the  aid  of  the  fossils  which 
they  contain,  to  identify  a  number  of  more  or  less  well  denned 
epochs  in  the  geological  history  of  the  earth. 

At  the  bottom  of  the  stratified  series  lies  an  immense  thickness 
of  rocks  which  seem  to  have  been  originally  sedimentary, 
but  many  of  which  have  undergone  profound  changes  due  to 
pressure  and  heat  since  they  were  laid  down.  These  rocks 
indicate  a  very  long  period  of  the  earth's  history,  the 

1  Except  in  a  few  cases  where  contortion  has  been  so  extensive  as  to  reverse  the 
relative  positio 


GEOLOGICAL  TIME-SCALE,  AS  INDICATED  BY  THE  STRATIFIED  ROCKS. 


Range  in  Time  of 
.  Animal  Groups. 

1 

a 

Epochs,  with  relative  duration 
indicated  by  thickness  of 
stratified  rocks. 

Some  representative 
British  Formations. 

^    i 
ijiljl 

Illllll 

Recent  and  Pleistocene  4000  leet. 

Alluvium,  Peat,  ooulder  Clay,  &c. 

£ 

Pliocene  13000  feet. 

Norwich  and  Red  Crag,  Coralline 
Crag,  &c. 

::::::       ? 

| 

:S 

Miocene  14000  feet. 

b 

e 

1 

o 

Oligocene  12000  feet. 

' 

Hamstead,    Bejnbridge,    Osborne 
and  Headon  Beds. 

3i 

Eocene  20000  feet. 

Barton  and  Bagshot  Beds,  London 
Clay,  Thanet  Sands,  &c. 

t 

/ 

) 

Chalk  Greerisand   Gault  Wealdcn 

"a 

Cretaceous  44000  feet. 

&c.  '        / 

T£ 

b 

0 

O 

i 

Jurassic  8000  feet. 

Oolites,  Lias. 

& 

Triassic  17000  feet. 

White  Lias  (Rhsetic),  Elgin  Sand- 
stone,  &c. 

Permian  12000  feet. 

Magnesian         Limestone,       Red 
Marls,  &c. 

Carboniferous  29000  feet. 

Coal    Measures,    Millstone    Grit, 
Mountain  Limestone,  &c. 

£ 

s_, 
0 

Devonian  22000  feet. 

Old  Red  Sandstone. 

o 

M 

Silurian  15000  feet. 

Ludlow  and  Wenlock  Shales  and 
Limestones,  &c. 

£ 

Ordovician  17000  feet.  . 

Slates,  Flags,  Sandstones,  &c.,  of 
Bala,  Caradoc,  Llandeilo,  Skid- 
daw,  &c. 

| 

Cambrian  26000  feet. 

Lingula  Flags,  Harlech  Series,  &c. 

Pre-Cambrian  (extent  unknown). 

i 

Archaean     Schists,     Gneiss     and 
other  Crystalline  Rocks  ;  largely 
metamorphic. 

' 

THE  AGE   OF   THE   HABITABLE   EARTH        285 

pre-Cambrian  epoch,  during  which  organic  evolution  must  have 
been  taking  place,  though  only  somewhat  doubtful  indications 
of  organic  remains  have  been  met  with.  Probably  the  pre- 
Cambrian  rocks  originally  contained  abundant  fossils,  which  have 
been  destroyed  by  the  treatment  to  which  they  have  been  sub- 
jected. All  the  subsequent  periods  of  the  earth's  history, 
however,  are  represented  by  rocks  containing  organic  remains  in 
greater  or  less  abundance.  These  deposits  indicate  a  division  oi 
the  earth's  history  since  pre-Cambrian  times  into  three  main 
'  eras — primary  or  palaeozoic,  secondary  or  mesozoic  and  tertiary 
or  cainozoic.  Each  of  these  three  is  subdivided  into  a  series  of 
epochs,  as  shown  in  the  accompanying  table,  in  which  are  also 
mentioned  some  of  the  principal  rock  formations  by  which  the 
different  epochs  are  represented  in  the  British  Islands. 

The  thickness  of  the  strata  deposited  during  each  period  differs  f 
greatly  in  different  parts  of  the  world,  and  in  North  America  the 
various  formations  appear  to  be  much  better  developed  than  in 
North  Western  Europe.  It  is  of  the  utmost  importance  that  our 
estimates  of  this  thickness  should  be  as  accurate  as  possible,  for 
they  constitute  important  data  from  which  to  calculate  the  age 
of  the  habitable  earth  and  the  duration  of  the  different  geological 
periods. 

Sir  Archibald  Geikie,1  in  1892,  very  cautiously  estimated  the 
total  thickness  of  the  stratified  rocks,  where  most  fully  developed, 
from  the  bottom  of  the  Cambrian  onwards,  as  not  less  than 
100,000  feet.  He  also  estimated  that  it  would  take  from  730  to 
6,800  years  to  add  a  foot  to  this  thickness  by  accumulation  of 
material  derived  from  the  denudation  of  the  land.  Of  course  the 
actual  time  must  vary  very  greatly  according  to  the  nature  of  the 
material  deposited  and  the  agencies  at  work.  There  is,  however, 
no  reason  to  suppose  that  denudation  and  sedimentation  took  place, 
on  an  average,  more  rapidly  in  past  geological  time  than  at  the 
present  day.  If,  then,  we  take  the  average  between  the  most 
rapid  and  the  slowest  rate  of  denudation  and  sedimentation,  and 
the  thickness  of  the  stratified  rocks  as  calculated  by  Professor 
Geikie,  we  arrive  at  376,500,000  years  as  the  age  of  the  habitable 
earth  exclusive  of  the  pre-Cambrian  period. 

Sir  Archibald  Geikie  appears,  however,  to  have  assumed  for 
the  purposes  of  his  calculation  that  material  derived  from  the 
wearing  away  of  the  land  has  been  spread  out  over  an  equal  area 

1  Presidential  Address,  British  Association,  Edinburgh,  1892. 


286       OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

of  sea-bottom,  so  that  every  foot  in  depth  worn  off  the  land  is 
taken  to  indicate  a  foot  in  thickness  piled  up  in  new  deposits. 
This  hardly  seems  to  be  a  justifiable  assumption,  for  in  many 
localities  sediment  derived  from  a  large  area  must  be  accu- 
mulated over  a  small  one,  and  the  increase  in  thickness  of  the 
newly  forming  rock  must  therefore  take  place  at  a  much  greater 
rate  than  the  decrease  in  thickness  of  that  which  is  being  worn 
away.  / 

Professor  Sollas,  in  his  presidential  address  to  the  Geological 
Society  of  London  in  1909,  gives  the  total  maximum  thickness  of 
the  sedimentary  rocks  from  the  bottom  of  the  Cambrian  to  the  top 
of  the  most  recent  formations  as  being  no  less  than  253,000  feet. 
I  have  accepted  this  estimate  in  drawing  up  the  geological  time- 
scale  on  page  284,  in  which  the  supposed  thickness  of  the 
deposits  belonging  to  each  of  the  main  subdivisions  of  the  three 
great  eras  is  indicated  both  numerically  and  by  the  relative  depth 
of  the  space  allotted  to  each. 

Professor  Sollas,  however,  estimates  a  much  more  rapid  rate  of 
accumulation  of  these  deposits — as  high,  indeed,  as  one  foot  per 
century — which,  even  with  his  own  increased  estimate  of  thickness, 
would  give  only  25,800,000  years  as  the  total  lapse  of  time  since 
the  commencement  of  the  Cambrian  epoch.  But  he  also  points 
out  that  there  is  good  reason  for  believing  that  the  pre- 
Cambrian  epoch  must  have  been  as  long  as  all  the  succeeding 
epochs  put  together,  and  the  total  lapse  of  time  since  the  formation 
of  the  stratified  rocks  (including  the  pre-Cambrian)  commenced 
might  accordingly  be  estimated  at  50,600,000  years. v 

We  must  also,  of  course,  make  abundant  allowance  for  the  fact 
that  denudation  has  been  going  on  at  all  times  since  the  deposition 
of  the  stratified  rocks  commenced,  and  that  the  total  thickness  of 
these  rocks  has  been  enormously  reduced  thereby,  the  same 
material  having  been  used  over  and  over  again  in  rock  formation. ' 

Where  opinions  differ  so  much,  and  where  there  is  so  much 
uncertainty  as  to  the  value  of  the  data  themselves,  it  is  obvious 
that  any  estimate  of  geological  time  derived  from  the  thickness 
of  the  sedimentary  rocks  can  have  only  a  doubtful  value,  and  that 
it  is  highly  desirable  that  such  an  estimate  should  be  checked 
by.  making  use  of  other  sources  of  information. 

Several  other  methods  of  estimating  the  age  of  the  earth  have 
been  employed.  Perhaps  the  most  satisfactory  is  that  which  is 
especially  associated  with  the  name  of  Professor  Joly,  and  which 


THE   AlGffi   OF   $HE,  HABITABLE   EARTH      (287$ 


depends  upon  the  amount  of  sodium  present  in  solution  in  sea- 
water.  If  we  knew  the  total  amount  of  sodium  present  in  the 
ocean,  and  if  we  also  knew  the  rate  at  which  it  has  been  accu- 
mulating since  the  ocean  was  first  formed,  it  would  be  a  simple 
matter  to  calculate  the  age  of  the  ocean  and  therefore  that  of 
the  stratified  rocks,  whose  existence  must  have  commenced  with 
that  of  the  ocean  itself.  The  greater  part  at  any  rate  of  the 
salt  contained  in  the  sea  must  have  been  derived  from  the 
disintegration  of  the  land  and  carried  down  by  rivers.  Estimates 
have  been  made,  based  upon  careful  analysis,  of  the  amount  of  ^ 
sodium  contained  in  the  sea  and  in  the  water  of  various  rivers, 
as  well  as  of  the  amount  of  water  which  flows  into  the  sea 
annually,  and  Professor  Sollas  has  come  to  the  conclusion  that 
"  on  a  review  of  all  the  facts,  the  most  probable  estimate  of  the 
age  of  the  ocean  would  appear  to  lie  between  80  and  150  millions 
of  years,"  which  is  considerably  in  excess  of  the  estimate  which 
he  arrives  at  from  the  consideration  of  the  thickness  of  the 
sejjimentary  rocks. 

S^NQ  cannot  assume,  however,  that  the  earth  has  been  habitable 
during  the  whole  of  this  period,  for  when  the  ocean  was  first 
formed  both  it  and  the  land  must  have  been  at  far  too  high  a 
temperature  to  admit  of  the  existence  of  protoplasmic  organisms. 
Possibly  if  we  assume  100,000,000^years  as  the  total  age  of  the 
habitable  earth,  and  therefore  as  the  time  which  has  been  available 
for  the  evolution  of  the  organic  world,  we  shall  be  as  near  as  we 
can  get  to  the  truth  in  the  present  state  of  our  knowledge.1 

We   must   now   turn   to   the  consideration   of   the  geological 
record  itself,  and  in  the  first  place  we  may  ask,  what  is  to  be 
legitimately  expected  from  such  a  record  ?     Have  we  any  right 
to   expect   anything  like   a   complete   representation,  by   fossil 
remains,  of  the  past  history  of  the  organic  world  ?     Obviously 
the  answer  to  this' question  must  be  a  decided  negative.     The 
imperfection  of  the  geological  record  is  due  to  many  causes.     It 
must  have  been  imperfect  when  first  laid  down,  for  the  great    ^ 
majority  of    organisms    usually    disintegrate   and    pass   away 
without  leaving  any  recognizable  remains  behind  them  at  all. 
In  most  cases  it  is  only  hard  skeletal  structures  that  have  any  * 
chance  of  being  preserved.      Bodies  composed  entirely  of  soft   ^/ 
tissues,  such  as  jelly-fish  and  many  worms,  are   rarely  repre- 
sented   by   fossils.     Even    hard    structures,    such    as    animal   ./ 
skeletons,    will   only    be  fossilized    if    they    happen   by    some 

1  Arrkenius  ("  Worlds  in  the  Making,"  pp.  42, 152)  suggests  1,000,000,000  years. 


288        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


fortunate  chance  to  find  their  way  to  some  place  where  rock 
formation  of  a  suitable  kind  is  going  on.  The  remains  of  land 
animals  may  be  carried  by  rivers  to  some  sea  or  lake  and  buried 
in  a  suitable  accumulation  of  mud  or  sand,  but  it  is  more  likely 
that  they  will  not.  Marine  animals,  provided  they  have  hard 
skeletons,  have,  of  course,  many  more  opportunities  of  attaining 
a  lasting  monument,  and  we  often  find  great  thicknesses  of  rock, 
such  as  chalk  and  limestone,  composed  almost  entirely  of  their 
remains. 

Even   when   the    record   has    been    successfully  established, 
however,  it    is  liable  to  destruction  by  various  agencies.     The 


A.  B.  C. 

FIG.  134. — Three  Species  of  Trilobites.     (From  British  Museum  Guide.) 
A.,  Agnostus princeps ;  B.,  Olenus  cataractes  ;  C.,  Staurocephalus  murchisoui. 

Crocks  containing  it  may  be  uplifted  above  sea-level  and  planed 
right  away  by  sub-aerial  denudation,  or  they  may  be  sunk  so 
beneath  later  accumulations  as  to  be  profoundly  altered  by 
the  action  of  heat  and  pressure,  by  which  means  any  fossils 
which  they  contain  may  be  completely  destroyed.  Then,  again, 
we  must  remember  that  only  a  relatively  small  proportion  of 
the  earth's  crust  is  accessible  for  investigation.  Deep-lying 

Crocks  may  be  brought  to  the  surface  by  upheaval,  and  denudation 
of  overlying  strata,  or  they  may  be  reached  by  deep  mining,  but 
we  know  practically  nothing  of  the  strata  which  now  lie  beneath 
the  bed  of  the  sea,  and  even  the  dry  land  has  only  been  tentatively 
scratched  by  inquisitive  man  in  a  few  places. 

Even   were   the    geological   record   far    less   perfect   than    it 
actually  is,  therefore,  its  imperfection  could  not  legitimately  be 


THE    GEOLOGICAL   RECORD  289 

used  as  an  argument  against  the  theory  of  organic  evolution. 
From  the   nature   of   the  case   it   must   be  imperfect   and  the 
surprising  thing  about  it  is  that  it  should  have  yielded  so  much  ^ 
'evidence  as  it  has  done.     The  earlier  portions  of  the  record,  how-  \) 
ever,  have  apparently  been  completely  destroyed.     The  fact  that  f    , 
already  in  the   Cambrian-  epoch   highly  organized  invertebrate 
animals^  such  as  trilobites  (Fig.  134)  and  brachiopods,  were  in 
existence,  shows  us  that  evolution  must  then  have  been  going  on 
for  an  immensely  long  period.     This  period,  as  we  have  already 
indicated,  is  represented  by  the  pre-Cambrian  rocks,  in  which 
the  organic  remains  have  been  so  far  destroyed  as  to  be  only 
recognizable  in  a  few  doubtful  cases. 

The  investigation  of  what  remains  of  the  geological  record 
has,  however,  yielded  some  very  remarkable  and  conclusive 
results.  In  the  first  place  we  have  learnt  that  life  on  the  earth 
has  been  continuous  without  a  break  from  the  commencement 
of  the  Cambrian  epoch  to  the  present  day,  and  in  the  second 
place  we  have  learnt  that  the  higher  groups  of  animals  and 
plants  have  appeared  on  the  earth  in  exactly  the  order  which  we 
should  expect  on  the  assumption  that  each  has  arisen  from  some 
preceding  and  more  lowly  organized  ancestral  group. 

In  discussing  this  point  we  may  confine  our  attention  to  the 
Vertebrata,  the  evolution  of  which  seems  to  have  taken  place 
entirely  during  post-Cambrian  times.  The  most  lowly  organized 
group  of  true  vertebrates  existing  at  the  present  day  is  that  of 
the  Cyclostomata,  which  includes  the  lampreys  and  hag -fishes. 
These  are  distinguished  from  true  fishes  by  the  absence  of  jaws, 
the  mouth  being~suctoj^l,  and  by  other  primitive  features.  The 
skull  and  vertebral  column  remain  cartilaginous  throughout  life 
and  there  are  no  paired  limbs.  Neither  is  any  dermal  armour 
of  any  kind  developed.  The  only  structures  which  they  possess 
which  seem  at  all  likely  to  be  preserved  as  fossils  are  the  horny 
teeth.  Certain  bodies  known  as  conodonts,  which  occur  as 
fossils  in  strata  ranging  from  the  Lower  Silurian  to  the 
Carboniferous,  may  perhaps  represent  such  teeth.  If  so  they 
are  perhaps  the  earliest  vertebrate  remains  as  yet  known.1 

The  remarkable  group  of  fishes  known  as  Ostracodermi, 
however,  makes  its  appearance  in  the  Upper  Silurian  •  and 
continues  on  to  the  Upper  Devonian.  These  animals  agree 

1  Palaeospondylus,  from  the  Lower  Old  Red  Sandstone  of  Scotland,  is  probably 
also  a  cyclcstome.  ^ 

B.  U 


290        OUTLINES   OF   EVOLUTIONARY    BIOLOGY 

with  the  cyclostomes  in  the  absence  of  true  jaws  and  (in  most 
known  forms)  of  paired  fins,  and  in  the  extremely  primitive 
character  of  the  vertebral  column.  They  had  also,  like  the 
cyclostomes,  median  fins,  but  they  differed  from  the  latter  con- 
spicuously in  the  presence  of  a  strongly  developed  dermal  armour, 


FIG.  135. — ±veai\ji.«.aon  of  CephaJaspis  murchiioni,  from  the  Lower  Old  Red 
Sandstone,  X  £.  (From  British  Museum  Guide,  -after  Smith  Wood- 
ward.) 

bo  the  existence  of  which  their  fossil  remains  owe  their  preser- 
vation. As  representative  genera  of  this  group  we  may  men- 
tion Cyathaspis  (Upper  Silurian),  Pteraspis  (Lower  Devonian), 


FIG.  136. — Pterichthys  miUtri,  from  the  Lower  Old  Eed  Sandstone ;  restored 
b*  E.  H.  Traquair.  A.  Dorsal ;  B.  Ventral ;  0.  Lateral  views,  X  £. 
(From  Smith  Woodward's  "Vertebrate  Palaeontology.") 

Cephalaspis  (Upper  Silurian  and  Lower  Devonian,  Fig.  135)  and 
Pterichthys  (Lower  Devonian,  Fig.  136). 

The  origin  of  the  Ostracodermi  is  unknown,  but  their  relation- 
ship to  the  cyclostomes  has  led  to  their  inclusion  with  the  latter  in 
one  class,  the  Agnatha  (jawless  vertebrates),  which  stands  at  the 


THE   GEOLOGICAL   RECORD 


201 


very  bottom  of  the  vertebrate  series  and  was  probably  derived  from 
some  primitive  chordate  ancestor,  not  unlike  Amphioxus,  which 
had  no  hard  structures  to  be  handed  down  to  us  in  a  fossil 
condition.  It  is  true  that  the  ostracoderms  bear  a  curious 
resemblance  to  certain  extinct  contemporaneous  arthropods,  the 
Eurypteridse  (Fig.  137), 
but  this  resemblance  is 
probably  merely  super- 
ficial and  due  to  conver- 
gence owing  to  similar 
conditions  of  life;  it 
cajnnot  ha  accepted  as 
evidence  of  the  origin  of 
vertebrates  from  arthro- 
podan  ancestors,  which, 
on  other  grounds,  is  ex- 
tremely improbable. 

The  most  primitive 
group  of  true  jaw-bearing 
fishes  is  that  of  the  Elas- 
mobranchii,  including  the 
existing  sharks,  dog-fishes, 
skates  and  rays.  These 
have  well  developed  paired 
fins  and  a  dermal  armour 
of  scales  or  denticles,  but 
their  primitive  character 
is  shown  by  the  fact 
that  the  internal  skeleton 
remains  cartilaginous 
throughout  life.  Frag- 


FIG.  137. — Pttrygotus  osiliensis,  an  Upper 
Silurian  Eurypterid,  dorsal  view,  re- 
duced. (From  Wood's  "  Palaeontology," 
after  Schmidt.) 


mentary     indications     of 

elasmobranchs   occur 

already     in     the     Upper 

Silurian,  but  the  first  satisfactory  specimens  have  so  far  been  met 

with  in  rocks  of  Lower  Devonian  age  (Old  Red  Sandstone).     A- 

carboniferous  form,  Acanthodes,  is  represented  in  Fig.  188. 

The  remarkable  Dipnoi,  feebly  represented  by  the  mud- 
fishes or  lung-fishes  of  the  present  day — the  Australian  Neo- 
ceratodus  (Fig.*  110),  the  African  Protopterus  and  the  South 
American  Lepidosiren — seem  to  have  arisen  during  the 

u  2 


292        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Devonian  .period    and    to   have    attained    their    maximum    of 

development  and  specialization  during  that  epoch.     This  group 

is  of  especial  interest,  as  we  have  already  had  occasion  to  notice, 

as  indicating  how  the  transition  from  an  aquatic  to  a  terrestrial 

1  mode  of  life  was  first  rendered  possible  to  vertebrate  animals  by 

I  the  conversion  of  the  swim-bladder  into  a  pair  of  lungs.     The 

dipnoids,  however,  have  remained  aquatic  in  habit,  and  retain 

their  gills  for  aquatic  respiration  as  well  as  being  able  to  breathe 

air  directly  by  means  of  their  lungs. 

The  ordinary    bony   fishes   (Teleostei),   to    which    the    vast 

majority  of  existing  species  belong,  do  not  attain  any  prominence 

as  a  group  before  the  secondary  era,  but  they  had  fore-runners 

in    some  of  the   early    ganoids    which    date    back    to     lower 

•N     Devonian  times.     The  teleosts   are   the  most  specialized  of  all 


3     FIG.  138. — Restoration  of  AcantJiodes  wardf,  from  the  Coal  Measures  of 
^J  Staffordshire,  X  |.     (From  British  Museum  Guide.) 

* 

f  fishes  and  it  is  not  from  such  a  group  that  we  should  expect  the 
next  great  advance  in  organization  to  take  its  origin. 

There  are  the  strongest  anatomical  and  embryological  grounds 
for  believing  that  the  Amphibia,  a  group  which  at  the  present 
/  day  includes  the  frogs,  toads,  newts  and  salamanders,  are 
the  descendants  of  fish-like  ancestors  which  became  adapted  to 
an  air-breathing  mode  of  life  by  the  development  of  lungs.  The 
dipnoid  fishes,  which,  as  we  have  already  seen,  attained  their 
maximum  of  development  in  the  Devonian  epoch,  show  us 
clearly  enough  how  such  a  transition  probably  took  place,  and 
it  was  doubtless  either  from  some  dipnoid  form,  or  from  some 
other  primitive  type  of  fish  which  had  also  succeeded  in  con- 
verting its  swim-bladder  into  lungs,  that  the  Amphibia  arose. 
The  fact  that  amphibian  remains  first  occur  in  Lower  Carboni- 
ferous rocks  is  in  complete  harmony  with  this  hypothesis  as  to. 
the  origin  of  the  group. 


THE   GEOLOGICAL   RECORD 


293 


The  earliest  known  amphibians  are  the  Stegocephalia,  so- 
called  on  account  of  the  strongly  developed  bony  armour 
which  covered  the  head  and  formed  the  roof  of  the  skull,  as  it 
did  already  in  many  of  the  earlier  fishes.  These  Stegocephalia 
included  both  small,  salamander-like  forms  such  as  Branchio- 
saurus  (Fig.  139)  and  large  crocodile-like  creatures — the 
labyrinthodonts.  Soine  of  them  survived  into  the  Triassic 


FIG.  139. — Branrhiosaurus  amllystomus ;  restoration  of  Skeleton  (A)  and 
ventral  Armour  (B)  by  H.  Credner,  nat.  size.  Lower  Permian.  (From 
Smith  Woodward's  "  Vertebrate  Palaeontology.") 

epoch,  after  which  they  appear  to  have  become  entirely  extinct, 
and  although  a  fair  number  of  Amphibia  still  exist  at  the  present 
day,  the  group  must  be  regarded  as  having  attained  its  maximum 
of  development  in  late  Palaeozoic  and  early  Mesozoic  times.  The 
existing  frogs  jand  toads  are  very  highly  specialized  forms, 
completely  adapted  to  a  terrestrial  life  in  the  adult  condition  but 
for  the  most  part  resorting  to  water  to  deposit  their  eggs,  which 
hatch  out  in  the  form  of  aquatic  fish-like  larvae  or  tadpoles, 


OUTLINES   OF   EVOLUTIONABY  BIOLOGY 


breathing  by  means  of  gills  (Fig.  121).  The  newts,  on  the  other 
hand,  though  developing  lungs,  retain  the  fish-like  form  and  the 
aquatic  habit  throughout  life. 

V  In  one  respect  the  imperfection  of  the  geological  record  as 
regards  the  origin  of  the  Amphibia  from  fishes  is  especially  to 
be  regretted.  One  of  the  chief  distinctions  between  the  two 
groups  lies  in  the  fact  that  whereas  the  Amphibia  always 
have  pentadactyl  limbs  (except  when  these  have  been  lost 
by  degeneration),  the  fishes  have  only  fins,  which  are  not 
pentadactyl.  The  conversion  of  the  fin  into  a  pentadactyl  limb 

\was  no  doubt  an  adaptation  to  the  terrestrial  mode  of  life,  but 
neither  comparative  anatomy  nor  embryology  nor  yet  the  evidence 
of  the  geological  record  have  so  far  enabled  us  to  discover  how  the 
transformation  of  the  one  type  of  limb  into  the  other  actually  took 
place.  We  can  only  hope  that  light  may  yet  be  thrown  upon  this 
difficult  problem  by  the  discovery  of  the  fossil  remains  of  inter- 
mediate forms. 

The  reptiles  and  the  amphibians  are  closely  related  to  one 
another,  but  existing  reptiles  are  readily  distinguished  from 
existing  amphibians  in  a  variety  of  ways.  For  example,  they 
produce  much  larger  eggs,  within  which  the  young  are  nourished 
up  to  a  very  advanced  stage  of  development  by  means  of  the  yolk, 
while  the  embryo  is  provided  with  the  special  protective  and 
respiratory  fostal  membranes  known  as  amnion  and  allantois, 
which  are  not  found  in  amphibians.  A  reptile,  again,  never 
develops  gills  at  any  stage  of  its  existence,  while  an  amphibian 
not  only  has  gills  in  the  larval  but  sometimes  also  in  the  adult 
state. 

Fossil  remains  naturally  afford  no  clue  as  to  when  the  develop- 
ment of  the  foetal  membranes  and  the  suppression  of  the  larval 
gills  took  place,  and  we  are  thrown  back  entirely  upon  skeletal 
peculiarities  as  a  means  of  distinguishing  between  extinct 
members  of  the  two  groups.  From  one  point  of  view  this  is 
unsatisfactory,  but  the  very  difficulty  which  we  experience  in 
drawing  a  hard  and  fast  line  between  the  fossil  reptiles  and 
\  amphibians  only  serves  to  emphasize  the  fact  that  the  one  group 
has  been  derived  from  the  other. 

One  of  the  chief  osteological  peculiarities  which  distinguish 
existing  reptiles  from  amphibians  is,  as  Dr.  Smith  Woodward 
observes,  "  the  degeneration  of  the  parasphenoid  bone  and  its 
functional  replacement  in  the  basi-cranial  axis  by  the  pterygoids 


THE   GEOLOGICAL   RECORD 


295 


.     .     .     If  this  feature  in  the  palate  has  always  been  distinctive, 
Pahcohattcria,    from    the  Lower    Permian    of    Saxony,  is    the 
earliest  member  of  the  class  Reptilia  hitherto  discovered  ;  and  it 
is  certain  that  during  the  Upper  Permian  age  there  were  numerous 
reptiles  both  in   Europe  and  America,  probably  also  in  South 
Africa."1      So  far  as  the  evidence  goes,  therefore,  the  appearance  i 
of  the  Reptilia  succeeds  that  of  the  Amphibia  in  exactly  the  way  1 
demanded  by  the  theory  of  organic  evolution. 

The  reptiles  as  a  group  attained  their  maximum  of  develop- 
ment in  Mesozoic  times,  and  of  the  nine  great  orders  into 
which  the  class  has  been  subdivided  only  four  persist  at  the 
present  day,  and  one  of  these  four  is  represented  by  only  a  single 


FIG.  140. — Skeleton  of  Pariasaurus  laini,  X  3Y 

after  Seeley.) 


(From  Smith  Woodward, 


species.  This  species,  Sphenodon  (=  Hatteria)  punctatus  (Fig.  113), 
now  confined  to  certain  small  islands  off  the  coast  of  New 
Zealand,  is  the  sole  surviving  representative  of  the  order 
Rhynchocephalia,  and  not  only  constitutes  the  oldest  existing  type 
of  reptile  but  also  makes  a  very  close  approach  to  the  oldest  type 
known  from  fossil  remains — the  extinct  Pala3ohatteria  of  the  Lower 
Permian.  The  other  existing  orders  of  reptiles  are  the  Chelonia 
(turtles  and  tortoises),  the  Squamata  (lizards  and  snakes)  and  the 
Crocodilia  (crocodiles  and  alligators). 

Many  of  the  extinct  Secondary  reptiles  were  far  more  highly 
specialized  and  remarkable  than  any  existing  forms.  Different 
members  of  the  class  were  adapted  to  the  most  diverse  modes  of 
life,  the  group  having  taken  full  possession  of  sea,  land  and  air. 

1  A.  Smith  Woodward,  "  Outlines  of  Vertebrate  Palaeontology  for  Students  of 
Zoology  :'  (Cambridge,  1898). 


296         OUTLINES   OF   EVOLUTIONARY    BIOLOGY 


The  earliest  and  at  the  same  time  the  least  specialized  of  the 
extinct  orders  were  the  Anomodontia  or  Theromorpha,  which  Dr. 
Smith  Woodward  tells  us  were  "directly  intermediate  in  skeletal 


02 
'1° 

PH 


a '3 


characters  between  the  highest  Labyrinthodonts  (Mastodonsaurus 
and  its  allies)  and  the  lowest  Mammals  (Monotremata)."  These 
animals,  whose  remains  have  been  found  only  in  Permian  and 
Triassic  deposits,  were  terrestrial  creatures  with  limbs  modified 
for  walking  on  dry  land,  so  that  they  often  bore  a  close  general 


THE    GEOLOGICAL    RECORD 


297 


resemblance  to  mammals.  One  of  the  best  known  is  the  celebrated 
Pjiriasaurus  (Fig.  140),  described  by  the  late  Professor  Seeley 
from  the  Karoo  formation  of  Cape  Colony.  This  animal  attained 
a  length  of  some  ten  or  eleven  feet. 

The  Sauropterygia  or  Plesiosauria  (Fig.  141)  were  long- 
necked  lizard-like  forms,  sometimes  of  large  dimensions,  which 
had  become  re-adapted  to  a  marine  life,  and  whose  limbs,  while 
retaining  the  typical  pentadactyl  structure,  gradually  became 
modified  to  form  paddles.  The  group  seems^  to  have  persisted 


FIG.   143. — Skeleton  of   lyuanodon  lernissartensis,  as   reconstructed  by 
Dollo.  X  gV     (From  British  Museum  Guide.) 

throughout  the  whole  of  the  Secondary  period  and  shows  a  gradual 
evolution  from  the  Trias  onwards. 

The  Ichthyopterygia  or  Ichthyosauria  (Fig.  142),  which  also 
range  throughout  the  whole  of  the  Secondary  period,  were  more 
completely  adapted  to  life  in  the  ocean  and  acquired  a  remarkably 
fish-like  form,  like  the  whales,  dolphins  and  porpoises  amongst 
existing  mammals. 

The  Dinosauria  or  Ornithoscelida  formed  an  enormous  group 
of  land  reptiles  which  seems  to,  have  arisen  in  the  Triassic  per/od 
and  reached  its  climax  in  Jurassic  and  Cretaceous  times. 
Some  of  them  were  carnivorous,  others  herbivorous  ;  some  walked 
on  all  fours  and  others  on  their  hind  legs,  and  they  often  attained 
gigantic  dimensions.  The  well  Known  Iguanodon(Fig.l43),  from  the 


298  OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


THE    GEOLOGICAL   RECORD  299 

Weald  en  deposits  of  Europe,  was  a  huge  herbivore,  which  seems  to 
have  supported  its  massive  body,  chiefly  at  any  rate,  on  its  hind 
limbs,  which  were  much  more  strongly  developed  than  the  front 
pair.  The  American  Brontosaurus  (Fig.  144)  and  Diplodocus 
walked  on  all  fours,  and  the  latter  attained  a  length  of  80  feet, 
but  a  large  proportion  of  this  was  taken  up  by  the  enormously 
long  neck  and  tail.  In  both  these  forms  the  head  was  of 
astonishingly  small  dimensions  in  proportion  to  the  rest  of  the 
body.  Other  Dinosaurs,  such  as  Stegosaurus  (Fig.  145)  and 
Triceratops  (Fig.  146),  developed  an  extraordinary  dermal 
armature  of  bony  plates  or  spines. 

Perhaps  the  most  remarkable  of  all  the  extinct  reptiles  of 
the  Secondary  period,  however,  were  the  Ornithosauria  (Ptero- 
sauria  or  pterodactyls),  which  at  that  time  occupied  the  place  now 


FIG.  147. — Skeleton  and  Outline  of  Pteranodon  occidentalis,  from  the  Upper 
Cretaceous  of  Kansas,  U.S.A. ;  X  £%•    (From  British  Museum  Guide.) 

filled  by  the  birds  and  bats.  These  animals  were  very  perfectly 
adapted  for  flight,  the  wing  being  formed,  as  already  described  in 
Chapter  XVII,  by  an  extension  of  the  skin  supported  by  the  arms, 
legs  and  tail,  and  especially  by  the  enormously  elongated  fifth  digit 
(Fig.  99).  Some  of  these  flying  reptiles  attained  a  very  large 
size,  the  total  expanse  of  the  wings  in  Pteranodon  (Fig.  147) 
being  about  eighteen  feet. 

•    The  origin  of  birds  from   reptilian  ancestors  has  been  quite  Y 
conclusively    demonstrated    on    anatomical    and    embryolpgical 


FIG.   144. — Skeleton  of   Brontosaurus   excelsus,  from  the  Upper  Jurassic 

Wyoming,  U.S.A.;    X  ^.     (From  British  Museum  Guide,  after  O.  0. 

Marsh.) 
FIG.  145. — Skeleton  of  Steyosaurus  unyulatns,  from  Jurassic  of  Wyoming  ; 

X  eV      (From  Smith  Woodward's  "Vertebrate  Palaeontology,"  after 

O.  C.  Marsh.) 
FIG.  146. — Skeleton  of  Triceratops  prorsus,  from  Cretaceous  of  Wyoming; 

X  8V     (Frem  British  Museum  Guide,  after  O.  0.  Marsh.) 


300        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

grounds,  and  Professor  Huxley  included  both  birds  and  reptiles 
in  one  large  group,  the  Sauropsida.  The  structure  of  their 
wings  shows,  however,  that  birds  cannot  have  arisen  from 
pterodactyls  but  must  have  sprung  from  some  less  specialized 
form,  mainly  through  the  development  of  a  dermal  exoskeleton 
in  the  form  of  feathers,  which  constitutes  the  chief  distinguishing 
feature  of  the  group.  It  is,  therefore,  quite  in  accordance  with 
expectation  that  the  earliest  known  bird,  Archaeopteryx  (Fig.  151), 
should  have  been  found  in  the  lithographic  slates  of  Solenhofen 
in  Bavaria,  which  are  of  Upper  Jurassic  age,  and  should  exhibit 
characters  intermediate  between  those  of  existing  birds  and 
reptiles.  We  shall  have  occasion  to  refer  to  this  remarkable 
connecting  link  more  in  detail  in  our  next  chapter. 
-  The  Mammalia,  so  far  as  existing  evidence  indicates,  appeared 
at  an  earlier  date  than  the  birds,  but  the  earliest  known  mammals 
were  far  less  highly  specialized  forms  than  birds,  and  although 
what  appear  to  be  mammalian  remains  have  been  found  as  low 
down  as  the  Trias,  the  group  did  not  attain  much  importance 
i  before  the  Tertiary  epoch,  when  it  replaced  the  reptiles  as  the 
dominant  group  of  vertebrates.  Opinions  have  been  a  good  deal 
divided  as  to  whether  the  mammals  arose  directly  from  amphibian 
ancestors  or  from  some  primitive  group  of  reptiles.  On  the 
whole  the  evidence  seems  to  be  decidedly  in  favour  of  the  latter 
view,  which  fits  in  vpvy^wftll  with  the  first  appearance  of  the 
mammals  just  when  the  reptiles  were  beginning  to  become 
dominant,  and  with  the  fact  that,  as  we  have  already  seen, 
some  of  the  earlier  reptiles  were  extraordinarily  mammalian 
in  aspect. 

The  earliest  mammals  were  small  forms  of  doubtful  affinities. 
Dr.  Smith  Woodward  remarks  in  this  connection : — "  It  is  as  yet 
impossible  to  determine  at  what  particular  stage  in  the  evolution 
of  the  vertebrate  skeleton  the  lung-breathers  first  acquired  the 
characteristic  mammalian  circulatory  system,  the  milk-producing 
glands,  and  a  dermal  covering  of  hair,"  these  being  the  principal 
features,  apart  from  the  skeleton,  which  distinguish  mammals 
from  reptiles  at  the  present  day.  The  difficulty  is  increased  by 
the  fact  that  some  of  the  anoinodont  reptiles  had  acquired  a 
dentition  very  similar  to  that  of  primitive  mammals,  while  the 
earliest  known  mammals  are  represented  by  little  more  than  a 
few  fossil  jaws  and  teeth. 

The  Mammalia,  both  living  and  extinct,  may  be 'divided  into 


THE   GEOLOGICAL   RECORD  801 

three  main  subclasses.  (1)  Prototheria,  with  the  single  surviving 
order  Monotremata,  represented  by  the  Australian  spiny  ant- 
eater  (Echidna,  Fig.  92)  and  duck-billed  Platypus  (Ornithorhyn- 
chus,  Fig.  91),  which,  as  we  have  already  seen,  are  intermediate 
both  in  anatomical  structure  and  in  their  method  of  reproduction 
between  the  reptiles  and  the  more  typical  mammals.  (2)  Meta- 
theria,  with  the  sole  order  Marsupialia,  represented  to-day  by 
the  Australian  kangaroos,  wombats,  phalangers,  thylacines  and 
numerous  other  pouched  mammals,  and  by  the  American  opossums 
(Didelphyidae)  and  Csenolestes.  These  are  in  some  respects  primi- 
tive forms,  in  which  the  young  are  born  at  a  very  early  stage  of 
their  development  (there  being  at  the  most  only  a  very  feebly 
developed  placenta)  and  usually  carried  about,  attached  to  the 
teat  of  the  mother,  in  a  marsupium  or  pouch.  They  are  readily 
distinguished  by  osteological  features,  especially  the  structure  of  the 
jaws  and  teeth  (Fig.  108),  from  the  higher  mammals.  (3)  Eutheria, 
the  dominant  mammals  of  the  present  day,  with  a  large  number 
of  orders  —  edentates,  whales,  sirenians,  ungulates,  rodents, 
carnivores,  insectivores,  bats  and  primates ;  with  well  developed 
placenta  and  retaining  the  young  in  the  womb  until  a  very 
advanced  stage  of  development.  There  can  be  no  doubt  that 
the  Eutheria  are  on  the  whole  more  highly  organized  forms 
than  the  Metatheria  and  the  latter  than  the  Prototheria.  This 
is  especially  indicated  by  the  gradual  acquisition  and  perfection 
of  the  characteristically  mammalian  method  of  nourishing  the 
young  by  means  of  the  placenta  and  mammary  glands. 

The  order  of  appearance  in  time  of  the  principal  subdivisions 
of  the  class  Mammalia,  as  indicated  by  the  geological  record, 
entirely  supports  the  evolutionary  hypothesis.  The  earliest 
known  fossils  which  may  possibly  be  referable  to  the  class  are  of 
Triassic  age.  Tritylodon  is  represented  by  an  imperfect  skull 
from  the  Karoo  formation  of  South  Africa.  This  has  generally 
been  regarded  as  mammalian,  but  Dr.  Smith  Woodward,  notwith- 
standing the  fact  that  it  has  tuberculated  grinding  teeth  with 
deeply  cleft  roots,  considers  that  it  probably  belonged  to  an 
anomodont  or  theromorph  reptile.  Micro lestes  is  represented  by 
double-rooted,  multituberculate  teeth  obtained  from  European 
Rhaetic  formations,  and  may  also  possibly  be  reptilian. 

In  deposits  of  Lower  and  Upper  Jurassic  age  we  find  more 
convincing  evidence  of  the  existence  of  true  mammals.  The 
celebrated  Stonesfield  slate  of  Oxfordshire  (Lower  Jurassic)  has 


302        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

yielded  a  fragment  of  a  jaw,  containing  three  small  multituber- 
culate  grinding  teeth,  to  which  the  name  Stereognathus  has  been 
given,  and  one  of  the  Middle  Purbeck  beds  of  Swanage  (Upper 
Jurassic)  has  contributed  the  well  known  jaws  of  Plagiaulax 
(Fig.  148).  Remains  of  closely  related  forms,  supposed  to  have 
belonged  to  the  same  family  (Plagiaulacidse),  have  been  discovered 
in  Upper  Jurassic  and  Upper  Cretaceous  formations  of  North 
America,  and  the  family  appears  to  have  survived,  both  in  Europe 
and  North  America,  into  early  Tertiary  times. 

The  Plagiaulacidae  and  related  forms  have  been  grouped  together 
in  the  extinct  order  Multituberculata,  which  Dr.  Smith  Wood- 
ward places  provisionally  amongst  the  Prototheria,  side  by  side 
"  with  the  surviving  order  Monotremata,  no  remains  of  which  are 

known  to  occur  before  Tertiary 
times.  It  is  possible,  however, 
that  the  Multituberculata  may  be 
metatherian  rather  than  proto- 
therian. 

The  remains  of  undoubted  Meta- 
theria  (Marsupialia)  first  occur,  so 

FlG.  148.— Mandible  of  Plagiaulax     „  .       , , 

minor,  x  4.  (From  Smith  far  as  we  yet  know,  in  the  same 
Woodward's  "  Vertebrate  beds  as  the  earliest  Multituber- 
Paleontology,"  after  Fal-  culata  (jeaving  Qut  Qf  ac(Jount 

the  enigmatical  Tritylodon  and 

Microlestes).  Mandibles  of  Phascolotherium  (Fig.  149)  and 
Amphitherium  have  been  found  in  the  Stonesfield  slate,  while 
Triconodon  and  Spalacoth.erium  are  similarly  represented  in  the 
mammal  bed  at  Durdlestone  Bay  near  Swanage. 

Throughout  uhe  whole  of  the  Secondary  period  the  mammals 
remained  of  insignificant  size,  and  in  a  more  or  less  primitive 
condition,  such  as  is  represented  at  the  present  day  by  the 
surviving  monotremes  and  marsupials.  The  typical  placental 
mammals  (Eutheria)  are  notJ:nown  to  us  from  formations  of 
earlier  date  than  the  Eocene.  |Xhen,  all  at  once,  they  seem  to  have 
branched  out  in  every  direction  and  taken  possession  of  land,  sea 
and  air  just  as  the  reptiles  had  done  before  them ;  whales  replacing 
the  Ichthyosauria  and  Plesiosauria,  various  groups  of  land 
mammals  replacing  the  Theromorpha  and  Dinosauria,  and  bats 
sharing  with  the  birds  the  kingdom  of  the  air  which  had  formerly 
belonged  to  the  pterodactyls. 

As  in  the  case  of  their  amphibian  and  reptilian  predecessors, 


THE   GEOLOGICAL   EECOBD  303 

many  of  the  mammalian  groups  in  Tertiary  times  have  run  to 
great  size ;  most  of  the  larger  forms,  such  as  the  primitive 
ungulate,  Tinoceras  (Fig.  150),  of  the  American  Eocene,  and  the 
giant  ground  sloth,  Megatherium,  of  the  American  Pleistocene, 
are  already  extinct,  but  it  must  not  be  forgotten  that  the  existing 
whales  are  amongst  the  largest  animals  that  have  ever  lived,  and 
in  bulk  at  any  rate  will  bear  comparison  with  thje  largest  of 
the  great  extinct  reptiles. 

The  last  term  in  the  evolutionary  series  of  the  Mammalia  is 
man,  whose  advent,  so  far  as  we  at  present  know,  dates  back  only 


Nat.  size. 


FIG.    149.— Mandible  of    PlasfoUtlnrium  lucMandi ;    x   3.      (From  Smith 
Woodward's  "Vertebrate  Palaeontology,"  after  Goodrich.) 

to  about  the  commencement  of  the  Pleistocene  or   end  of  the 
Pliocene  epoch.1 

There  is  one  more  point  that  is  well  worth  emphasizing  about  the 
evolution  of  the  Vertebrata,  as  indicated  not  only  by  the  geological 
record  but  also  by  the  facts  of  comparative 'anatomy.  Each 
successive  great  group  appears  to  have  arisen,  not  from  the 
most  highly  specialized  members  of  some  preceding  great 
group,  but  from  comparatively  undifferentiated  forms.  Thus  the 
Amphibia  arose,  not  from  bony  fishes,  but  from  primitive  dipncids 
or  ganoids  ;  the  reptiles  arose,  not  from  frogs  or  toads,  but  from 
primitive  stegocephalian  amphibia ;  the  birds  arose,  not  from 
pterodactyls,  but  from  comparatively  unspecialized  reptiles  ;  the 
mammals  also  arose  from  the  more  primitive  reptilian  forms,  and 
man  himself,  whose  advent  undoubtedly  marks  the  commencement 
of  a  fresh  line  of  evolution,  belongs  to  the  order  Primates,  which 
in  respect  of  bodily  organization,  as  seen,  for  example,  in  the 

,  »    Vide  Chapter  XXVII. 


304         OUTLINES    OF    EVOLUTIONARY   BIOLOSY 


typical  pentadacfcyl  limbs,  is  far  more  primitive  than  many  other 
mammalian  groups. 

Each  great  group  seems  to  have  begun  in  a  small  way,  then 
developed  rapidly,  branching  out  in  many  directions  and  becoming 
the  dominant  group  for  the  time  being,  only  to  dwindle  away 
again  and  give  place  to  some  new  and  vigorous  off-shoot.  The 
dominance  of  any  particular  group  has  often  been  accompanied 
by  the  attainment  of  enormous  size  by  its  individual  members,1 


FIG.  150.— Skeleton  of  Twoceratingena,  from  the  Middle  Eocene  of  Wyoming, 
X  ^V     (From  British  Museum  Guide,  after  O.  C.  Marsh.) 
j 

and  it  is  not  impossible  that  this  may  h&ve  had  something  to  do 
with  its  subsequent  decline  or  complete  extinction. 

So  far  as  the  animal  kingdom  is  concerned,  we  may  perhaps 
say  without   exaggeration  that   the    succession  in  time   of    the 
different  groups,  as  indicated  by  the  geological  record,  amounts 
"NJ  to  positive  demonstration  of  the  truth  of  the  theory  of  organic 
evolution.     In  the  case  of  plants  the  record  is  perhaps  not  quite 
clear,  but  here  also  there  can  be  no  reasonable  doubt  that  the 
/great  groups  succeeded  one  another  in  a  manner  consistent  with 
^eo£^,    commencing  with    the    algae    and   ending  with  the 
flowering  plants  of  the  present  day. 

1  A  possible  explanation  of  this  fact  is  suggested  in  Chapter  XXVI. 


CHAPTEK  XX 

Fossil  pedigrees — Ancestry  of  birds,  horses,  elephants  and  whales. 

IN  our  last  chapter  we  gave  a  brief  outline  of  the  general  course 
of  evolution  amongst  vertebrate  animals  as  indicated  by  the 
geological  record.  We  may  now  study  in  somewhat  greater 
detail  certain  branches  of  the  great  phylogenetic  tree  which  are 
especially  well  represented  by  fossil  remains  and  therefore 
particularly  instructive  from  the  point  of  view  of  the  evolution 
theory. 

One  of  the  most  highly  specialized  groups  of  vertebrates  that 
have  ever  existed  is  that  of  the  birds.  We  have  already  pointed 
out  that,  on  anatomical  grounds,  birds  are  classed  together  with 
reptiles  as  Sauropsida.  They  agree  with  reptiles  in  their  method 
of  reproduction  by  means  of  large,  heavily  yolked  eggs,  and  in 
the  presence,  in  the  embryo,  of  the  characteristic  foetal  membranes, 
amnion  and  allantois,  as  well  as  in  certain  morphological  charac- 
ters of  the  adult.  They  differ  from  reptiles,  however,  in  many  4 
striking  features.  Thus  they  possess  feathers,  which  almost  (but 
not  quite)  completely  replace  the  reptilian  scales  as  a  protective 
exo-skeleton.  The  anterior  limbs  are  modified  to  form  wings 
constructed,  as  we  have  already  seen  in  Chapter  XVII,  on  an 
entirely  different  plan  from  those  of  flying  reptiles.  The  digits  of  L 
the  hand  are  very  greatly  reduced ;  only  one  of  them,  and  that  in 
a  vestigial  condition,  projects  freely  from  the  anterior  border  of  the 
wing,  forming  the  so-called  "  ala  spuria  "  (Fig.  99).  The  true  tail 
is  greatly  abbreviated  and  the  caudal  vertebrae  reduced  in  number 
and  to  a  large  extent  fused  together  to  form  the  "  ploughshare 
bone  "  which  supports  the  tail  feathers.  Lastly,  all  existing  birds 
have  completely  lost  their  teeth,  which  are  functionally  replaced 
by  the  horny  beak. 

The  remains  of  the  earliest  known  bird,  Archaeopteryx  (Fig.  151), 
have  been  found  in  the  celebrated  lithographic  stone  of  Solenhofen 
in  Bavaria,  of  Upper  Jurassic  age,  which,  owing  to  its  extremely 
fine  grain,  is  peculiarly  well  suited  for  the  preservation  of  even 

B.  x 


306        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

such  delicate  structures  as  feathers.  This  animal  was  about  the 
size  of  a  rook  and  the  presence  of  well  developed  feathers  and 
wings  of  the  avian  type  is  alone  sufficient  to  show  that  we  are 
dealing  with  a  true  bird.  It  still  exhibits,  however,  a  number  of 
features  which  are  usually  met  with  in  reptiles  but  have  dis- 
appeared in  modern  birds.  The  digits  of  the  anterior  limb  are 
not  nearly  so  much  reduced  as  in  the  latter,  for  three  claw-bearing 


FIG.  151. — FossiJ  Bemains  of  Archceopteryx  stemensi,  showing  the  three 
fingers  in  each  wing,  the  long  tail,  feathers,  &c.  (From  Lankester's 
"  Extinct  Animals.") 

fingers  project  from  the  anterior  margin  of  the  wing  ;  the  tail  is 
elongated  like  that  of  a  lizard  and  supported  by  about  twenty 
separate  vertebrae  each  carrying  a  pair  of  feathers ;  and  numerous 
teeth  are  present  in  the  beak. 

It  is  obvious  that  Archaeopteryx  represents  a  stage  in  the 
derivation  of  birds  from  reptilian  ancestors,  and  this  is  exactly 
what  we  should  expect  of  the  earliest  birds  in  accordance  with 
the  theory  of  evolution.  Unfortunately,  with  the  exception  of  a 
few  other  toothed  birds  of  Cretaceous  date,  Archaeopteryx  is 


EVOLUTION   OF  THE    HORSE 


307 


almost  the  only  link  in  the  pedigree  of  birds  which  has  so  far 
been  discovered,  and  it  teaches  us  nothing  as  to  the  origin  of 
those  characteristic  avian  structures,  the  feathers,  which  it 
possesses  already  in  a  fully  developed  condition. 

It  will  be  observed  that  Archaeopteryx  occupies  a  position 
between  reptiles  and  typical  birds  exactly  comparable  with  that 
of  the  Monotremata  between  reptiles  and  typical  mammals  (see 
Chapter  XVII),  the  only  difference  being  that  the  Monotremata 
still  survive  side  by  side  with  mammals  of  the  most  highly 
advanced  type,  while  Archaeopteryx  has  long  since  become  extinct. 

One  of  the  most  complete  fossil  pedigrees  as  yet  known  to  us 


FIG.  152.— Skeleton  of  Phenacodus,  a  five-toed  Eocene  Ungulate.     (From 
Lankester's  "  Extinct  Animals.") 

is  that  of  the  Equidae  or  horse  family.  As  we  have  already  seen 
in  Chapter  XVII,  the  study  of  comparative  anatomy  indicates  very 
clearly  that  the  highly  specialized  single-toed  limbs  of  the  horse 
(Fig.  97)  must  have  arisen  from  some  primitive  pentadactyl 
type  by  gradual  suppression  of  all  the  digits  except  the  middle 
one.  Amongst  the  fossil  remains  of  horse-like  animals  which 
abound  in  various  tertiary  formations  of  Europe  and  America  we 
find  a  very  complete  series  of  stages  in  the  evolution  of  the 
modern  horse,  which  entirely  confirms  this  conclusion.  Our 
knowledge  of  this  extremely  interesting  phylogenetic  series  is  due 
largely  to  the  late  Professor  0.  C.  Marsh  and  has  been  admirably1 
summarized  by  Mr.  E.  g.  Lull  in  the  American  Journal  of  Science. 

1  Series  IV,  Vol.  XXI II,  1907. 

•       x  2 


308        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 


FIG.  153.—  Outlines  of  Horses  of  different  Geological  Periods,  showing  their 
Relative  Sizes.     (From  Lull.) 

a,  Praborohippus  (Eocene) ;  6,  Oroliippus  (Eocene) ;  c,  Mesoliippus  (Oligocene) ;  d,  Meryc- 
liippus  (Miocene);  e,  Pliohippus  (Pliocene) i  /,  Equus  (Receut). 


EVOLUTION    OF    THE   HORSE  309 

It  is  generally  admitted  that  the  Equidse  originated  from  the 
Condylarthra,  a  group  of  primitive,  five-toed,  ungulate  mammals 
which  made  its  appearance  in  early  Eocene  times,  and  the  best- 
known  representative  of  which  is  Phenacodus  (Fig.  152).  The 
evolution  of  the  horses  appears  to  have  taken  place  chiefly  in 
America,  though  occasionally  representatives  of  the  group  seem  to 
have  migrated  to  or  from  Europe,  doubtless  by  a  former  land 
connection  in  the  neighbourhood  of  Behring  Strait.  In  com- 
paratively recent  times,  however,  the  family  became  confined  to 
the  old  world  and  was  only  re-introduced  to  America  by  human 
agency. 

The  course  of  their  evolution  has  evidently  been  determined  by 
the  development  of  extensive,  dry,  grass-covered,  open  plains 
on  the  American  continent.  In  adaptation  to  life  on  such  areas 
structural  modification  has  proceeded  chiefly  in  two  directions. 
The  limbs  have  become  greatly  elongated  and  the  foot  uplifted  | 
from  the  ground,  and  thus  adapted  for  rapid  flight  from  pursuing 
enemies,  while  the  middle  digit  has  become  more  and  more 
important  and  the  others,  together  with  the  ulna  and  the  fibula, 
have  gradually  disappeared  or  become  reduced  to  mere  vestiges. 
At  the  same  time  the  grazing  mechanism  has  been  gradually 
perfected.  The  neck  and  head  have  become  elongated  so 
that  the  animal  is  able  to  reach  the  ground  without  bending  its 
legs,  and  the  cheek  teeth  have  acquired  complex  grinding  surfaces 
and  have  greatly  increased  in  length  to  compensate  for  the 
increased  rate  of  wear.  As  in  so  many  other  groups,  the  evolution 
of  these  special  characters  has  been  accompanied  by  gradual/ 
increase  in  size  (Fig.  153).  Thus  Eohippus,  of  lower  Eocene 
times,  appears  to  have  been  not  more  than  11  inches  high  at  the 
shoulder,  while  existing  horses  measure  about  64  inches,  and  the 
numerous  intermediate  genera  for  the  most  part  show  a  regular 
progress  in  this  respect. 

All  these  changes  have  taken  place  very  gradually,  and  a 
beautiful  series  of  intermediate  forms  indicating  the  different! 
stages  from  Eohippus  to  the  modern  horse  (Equus)  have  been 
discovered.  The  sequence  of  these  stages  in  geological  time 
exactly  fits  in  with  the  theory  that  each  one  has  been  derived 
from  the  one  next  below  it  by  more  perfect  adaptation  to  the 
conditions  of  life.  Numerous  genera  have  been  described,  but  it 
is  not  necessary  to  mention  more  than  a  few. 

The  first  indisputably  horse-like  animal  appears  to  have  been 


810        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


Im 


Hyracotherium,  remains  of  which  have  been  found  in  the 
London  Clay  (Lower  Eocene).  Another  Lower  Eocene  genus 
was  Eohippus,  which  seems  to  have  arisen  in  Western  Europe, 
possibly  from  a  hyracotherian  ancestry,  and  migrated,  by  way 
of  Northern  Asia,  to  America,  where  its  remains  occur  in  rocks 
of  the  same  age.  In  this  animal  (Fig.  154)  the  fore  foot  had 
four  well  developed  digits  and  the  thumb  was  represented  by  a 
splint  bone;  in  the  hind  foot  the  great  toe  had  entirely  dis- 
appeared and  the  fifth  digit  was  represented  only  by  a  splint 

bone.  In  both  fore  and  hind  feet 
the  third  or  middle  digit  was  already 
conspicuously  larger  than  any  of  the 
others. 

Eohippus  was  succeeded  by  Pro- 
torohippus  (Fig.  153,  a),  which  was 
some  3  inches  higher  and  had  lost 
the  vestigial  thumb.  Then  came 
Orohippus  (Fig.  153,  b),  again  a  little 
larger  and  with  closely  similar  feet 
(Fig.  155),  but  with  a  considerable 
advance  in  the  evolution  of  the 
grinding  teeth.  The  last  of  the 
Eocene  horses  was  Epihippus,  still 

with  four  toes   in  front  and  three 
\i/ 

///  behind,   but    with   the   lateral   toes 

&  further  reduced  in  size  and  another 

1 54.  _  a>  Fore  Foot  and    distinct  advance  in  tooth  structure.  ~r" 
b,  Hind  Foot  of  Eohippus        jn  Oligocene  times  there  occurred 

in  North  America  Mesohippus  and 
Miohippus,  and  in  Europe  Anchi- 
therium.  Mesohippus  (Fig.  153,  c)  was  18  inches  or  more  in 
height,  with  three  digits  and  a  vestige  of  the  fifth  in  the  fore 
foot  and  three  digits  only  in  the  hind  foot  (Fig.  156).  Miohippus 
attained  a  height  of  24  inches  and  closely  resembled  Mesohippus 
in  the  structure  of  its  feet.  Anchitherium  is  supposed  to  be 
a  European  derivative  of  Miohippus. 

In  the  Miocene  period  the  horses  appear  to  have  attained  their 
maximum  of  development  as  a  group,  and  a  number  of  extinct 
American  genera  are  distinguishable.  Merychippus  (Fig.  153,  d), 
Protohippus  and  Neobipparion  were  still  three-toed  horses,  though 
the  lateral  digits  were  now  greatly  reduced  (Fig.  157).  Pliohippus 


pernix,   X  £.   .  (From  Lull, 
after  Marsh.) 


EVOLUTION   OF   THE    HORSE 


(Fig.  153, 6'),  which  continued  on  into  Pliocene  times  and  attained 

a  height  of  48  inches,  had  the  second  and  fourth  digits  of  each 

foot  represented  by  mere   splint   bones   as   in   modern  horses, 

and  had  therefore  already  attained  to  the  single-toed  condition 

(Fig.  158). 

In  Pliocene  times,  however,  we  still  find  a  three- toed  horse — 

Hipparion — surviving  in  Europe,  but  the  modern  one-toed  genus 

Equus  (Fig.   153,  /)  also  makes 

its  appearance  both  in  the  old  and 

new  worlds,  becoming  extinct  in 

the  new  world  in  Post-Pleistocene 

times    until    re-introduced    from 

Europe  by  the  agency  of  man. 

The  time  occupied  in  the  evolu- 
tion  of    the   genus   Equus   from 

its  remote  ancestor  Eohippus  is 

estimated  by  Professor  Sollas  at 

five  or  six  millions  of  years.     This 

period  is  sufficient  to  allow  of  a 

very   slow   and    gradual    change ' 

from  one  condition  to  the  other. 

Allowing    five     years    for     each 

generation,  Sollas  arrives  at  the 

conclusion  that  somewhere  about 
r\  a   million   generations   intervene/ 
' -between  the  two  extremes.     The 

total   increase   in   height    during 

this  time  has  been  53  inches,  and 

if  this  increase  were  spread  fairly 

uniformly  over  the  whole  period 

it   would   only  mean   about   0*00005   inch   for  each   successive 

generation — an  amount  which  would  be  quite  imperceptible  to 

human  observers. 

In  reconstructing  such  a  pedigree  as  that  of  the  horse  from 

palaeontological  evidence  it    is  of    course  necessary  to  bear  in 

mind  that  the  great  majority  of^extinct  forms  which  come  to 

light  will  almost  certainly  not  be  actually  in  the  direct  line  of 


IV 


IV 


FIG.  155. — a,  Fore  Foot  and  b,  Hind 
Foot  of  OroMppus  ayilis,  X  %. 
(From  Lull,  after  Marsh.) 


descent.     Collateral  branches  will  have  been  given  off  from  the'   >v 
phylogenetic  tree  in  various  directions,  and  it  is  much  more  likely 
that  any  particular  form  discovered  will  belong  to  one  of  these 
branches  than  that  it  will  belong  to  the  main  stem.     This  fact. 


312        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


-U 


-cb 


however,  by  no  means  vitiates  the  general  argument,  for  it  is  usually 
possible  to  pick  out  pretty  accurately  those  which  come  into  or 
near  to  the  direct  line,  and  even  the  collaterals  afford  valuable 
evidence  as  to  the  general  course  of  evolution.  We  may  safely 
say  that  the  palseontological  evidence  amounts  to  a  clear; 

demonstration  of  the  evolution/ 
of  the  horse  from  a  five-toec} 
ancestor  along  the  lines  indf 
cated  above. 

•  The  ancestry  of  the  elephants 
is  less  well  known  than  that 
of  the  horses,  but  recent  dis- 
coveries in  the  Egyptian  Ter- 
tiary formations,  which  we  owe 
especially  to  the  investigations 
of  Dr.  Andrews,  have  done 
much  to  elucidate  the  history 
of  this  remarkable  group  of 
mammals,  and  there  can  now 
be  no  doubt  as  to  the  main 
line  of  evolution  which  has  led 
up  to  the  existing  Proboscidea. 
In  some  respects  the  elephants 
have  remained  in  a  some- 
what primitive  condition,  as 
is  indicated  very  clearly  by  the 
[fact  that  all  the  digits  remain 
well  developed  in  both  fore  and 
hind  feet.  It  is  in  the  struc- 
ture of  the  head  that  they 
exhibit  a  high  degree  of  special- 
ization, marked  particularly  by 
the  elongation  of  the  snout  to 
form  a  long  prehensile  trunk,  by  the  enormous  development  of 
the  occipital  region  of  the  skull,  by  the  enlargement  of  the 
incisor  teeth  to  form  great  tusks,  by  the  shortening  of  the  jaws 
and  by  the  increase  in  size  and  complexity  and  the  reduction  in 
number  of  the  cheek  teeth.  These  changes  have  been  accom- 
panied by  a  huge  increase  in  the  size  of  the  entire  body,  so  that 
most  of  the  elephants  are  amongst  the  largest  of  known 
mammals,  whether  fossil  ov  recent. 


FIG.  1 56.— a,  Fore  Foot  and  Z>,  Hind 
Foot  of  Mesohippus  celer,  X  i. 
(From  Lull,  after  Marsh.) 


EVOLUTION   OF   ELEPHANTS 


813 


Like  the  horses,  the  elephants  pruhably  originated  from  that 
primjiiye^iuigulate  group,  the  Condylarthra.  The  earliest  known 
form  exhibiting  proboscidean  characters  is  Moeritherium,  a 
tapir-like  animal  whose  remains  have  been  found  in  the  Middle 
and  Upper  Eocene  deposits  of 
the  Egyptian  Fayum.  This  was 
a  comparatively  small  creature,/ 
about  as  large  as  a  Newfoundland 
dog.  It  probably  differed  but  little 
from  other  primitive  ungulates, 
but  the  skull  (Fig.  159,  1)  already 
shows  marked  proboscidean  ten- 
dencies. The  position  of  the  nasal 
bones,  away  back  from  the  tip  of 
the  snout,  indicates  that  there 
was  in  all  likelihood  a  short  pro- 
boscis. The  occipital  region  of 
the  skull  is  beginning  to  grow  up 
and  air  cells  are  beginning  to 
develop  in  the  bones.  The  second 
I  pair  of  incisor  teeth  in  each  jaw 
'are  enlarged  to  form  small  tusks 
and  the  hinder  cheek  teeth  are 
beginning  to  show  an  increase  in 
complexity  of  structure.  The  total 
number  of  teeth  however  (86)  is 
only  eight  short  of  the  full  typical 
mammalian  dentition. 

The  next  stage  is  represented 
by  Palaeomastodon  (Fig.  159,  2) 
from  the  Upper  Eocene  of  the  same 
region,  some  species  of  which  were 
little  larger  than  Moaritherium 
while  others  attained  almost 

elephantine  proportions.  In  this  genus  we  notice  a  strong 
accentuation  of  the  proboscidean  characters.  The  occiput  is  higher, 
the  nasal  opening  in  the  skull  further  back,  the  upper  tusks 
better  developed,  the  cheek  teeth  more  complex ;  while  the  canines 
and  all  the  incisors  except  the  tusks  in  both  jaws  have  dis- 
appeared. It  will  be  observed  that  as  yet  there  is  no  shortening 
Of  the  jaws,  but,  on  the  contrary,  the  lower  jaw  has  become 


FIG.  157.— a,  Fore 
Hind  Foot  of 
whitneyi,  X  ^. 


Foot  and  b, 
Neohipparion 
(From  Lull.) 


314        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


considerably  elongated,  apparently  serving  as  a  support  for  the 
lengthening  proboscis. 

In  Tetrabelodon  angustidens,  from  European  Miocene  formations, 
.this  elongation  of  the  mandible  is  much  more  marked,  so  that 
I  the  lower  jaw  is  much  longer  than  the  upper  one  and  the  short 
_  lower  tusk  comes  to  project  almost  as 

far  forward  as  the  long  upper  one  (Fig. 
159,  3).  From  this  time  onwards,  how- 
ever, the  chin  shortens,  thereby  allowing 
greater  flexibility  to  the  proboscis,  so  that 
in  the  lower  Pliocene  we  find  Tctrabelodon 
longirostris  (Fig.  159,  4)  with  the  lower 
jaw  only  a  little  longer  than  the  upper, 
leading  the  way  to  the  mastodons  and 
true  elephants  (Elephas),  which  also 
appeared  in  Pliocene  times  and  in  which 
the  tusks  have  entirely  disappeared  from 
the  greatly  abbreviated  mandibles  while 
the  cheek  teeth  have  become  enormously 
enlarged  and  complicated  (Fig.  159;  5). 

We  have  here  a  wonderfully  perfect 
series  of  connecting  links  between  the 
most  primitive  known  ungulate  mammals 
and  the  elephants.  Only  forms  which 
appear  to  lie  in  or  near  the  direct  line  of 
descent  have  been  mentioned  in  the  above 
brief  account.  Other  modifications  of 
the  proboscidean  type  arose  as  lateral 
oft'shoots  from  this  main  stem.  One  of 
the  most  remarkable  of  these  is  Dino- 
therium,  with  its  great,  downwardly 
directed  lower  tusks  (Fig.  160),  which 
appeared  in  Europe  in  the  Pliocene  period. 
In  the  case  of  the  Cetacea,  a  group  which  includes  the  whales, 
porpoises  and  dolphins,  we  have  as  yet  only  a  much  more  frag- 
mentary pedigree,  but  still  quite  sufficient  to  justify,  on  the 
palaeontological  side,  the  conclusion,  already  arrived  at  on  ana- 
tomical grounds,  that  these  extremely  aberrant  forms  are  the 
f  descendants  of  typical  terrestrial  mammals  which  have  become 
re-adapted  to  an  aquatic  life  and  in  accordance  therewith  have 
re-acquired  a  superficial  resemblance  to  their  much  more  remote 


FIG.  158.— a,  Fore  Foot 
and  />,  Hind  Foot  of 
Plioliippuspernix,  X  |. 
(From  Lull.) 


EVOLUTION   OF    ELEPHANTS 


315 


Recent 
Pleistocene 
Unner  Pliocene 


ELEPHAS 
(short  chin) 


Lower  Pliocene      TETRABELODON 

[LONGIROSTRIS  STAGE] 
Unner  Miocene         (shortening  chin) 


Middle  Miocene     TETRABELODON 

[ANGUSTIDENS  STAGE] 
Lower  Miocene          (tonjchm) 


Unner  Oligocene 


into  Europe  -Asia 


Lower  Oligocene'} 

|>    PALAEOMASTODON 


Unner  Eocene 


c]  chin) 


M,,,    „  }    MOERITHERIUM 

Middle  Eocene   \  » 

7        (short  chin) 

Lower  Eocene  > 

Tiu.  159.— Some  Stages  in  the  Evolution  of  the  Skull  in  the  Proboscidea. 
(From  British  Museum  Guide.) 


316        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


fish-like  ancestors.1     The  fore  limbs  have  become  converted  into 
paddles  while  the  hind  limbs  have  entirely  disappeared  externally 


FIG.  160. — Skull  of  Dinotherium  giganteum,  Lower  Pliocene,  X  TV     (From 
Smith  Woodward's  "  Vertebrate  Palaeontology,"  after  Kaup.) 

(Fig.  161).     The  tail  has  become  flattened  out  into  a  horizontal 
\\fin  and  there  is  frequently  a  well  developed  dorsal  fin.   The  skull 


FIG.  161.— The  Dolphin,  Delphinita  delplm, 

Guide.) 


(From  British  Museum 


has  undergone  very  curious  changes.  The  brain  case  is  rounded 
and  strongly  arched  (Fig.  162)  and  the  nasal  apertures  or  blow- 
holes lie  far  back,  at  or  near  the  highest  point  of  the  he 

I  Compare  Chapter  XVII. 


ad 


EVOLUTION  OP  WHALES 


317 


((Fig.  101,  6),  while  the  jaws  have  become  greatly  elongated.    The 
teeth  have  in  some  cases  completely  disappeared,  as  in  the  whale- 


FIG.  162.— Skull  of  the  Dolphin,  X  £.     (From  British  Museum  Guide.) 

bone  whales  (except  for  total  vestiges),  while  in  others  they  are  \ 
present  in  large  numbers  but  have  lost  the  typical  mammalian 


FlG.    163.— Dorsal  and  lateral    views  of  the  Skull  of   a  primitive  Whale, 
Prozeuglodon  atrox,  X  ^T.     (From  British  Museum  Guide.) 

differentiation  into  incisors,  canines,  premolars  and  molars,  being 
represented  by  a  continuous  and  uniform  series,  all  of  which  are 
conical  in  shape  and  single-rooted.  Such  teeth  occur  in  both 


318       OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

jaws  of  the  porpoises  and  dolphins  (Fig.  162)  and  in  the  lower 
jaw  only  of  the  sperm  whale. 

In  the  extinct  shark-toothed  Dolphins  (Squalodontidae),  whose 
remains  have  been  found  in  Miocene  formations  of  Europe  and 
America,  the  teeth  are  still  differentiated  into  incisors,  canines, 
premolars  and  molars,  and -the  molars  have  double  roots  and 
compressed  crowns  with  serrated  edges. 

Further  back,  in  Eocene  times,  there  existed,  widely  distributed 
over  the  northern  hemisphere,  a  group  of  still  more  primitive 
whale-like  animals  known  as  Zeuglodontidse.  In  these  the  seven 
vertebrae  of  the  neck,  which  in  existing  whales  are  more  or 
less  fused  together  into  a  solid  mass  (Fig.  103),  are  all  separate, 
and  the  typical  dental  formula  is  identical  with  that  of  primitive 

,..31  4         3—2 

carnivorous  land  mammals,  viz.  i  ^,  c.  -=-,  p.m.  -,  m.     Q    . 

o         1  4  o 

The  genus  Prozeuglodon,  from  the  Egyptian  Eocene,  ap- 
proaches so  closely  in  the  characters  of  the  skull  and  teeth 
(Fig.  163)  to  the  primitive  carnivores  (Creodontia)of  about  the  same 
period  as  to  leave  no  reasonable  doubt  about  the  derivation  of  the 
Cetacea  from  that  group,  although  it  isjjviitft  possible  that  none 
of  the  extinct  forms  so  far  discovered  are  actually  in  the  direct 
line  of  descent  of  any  of  the  modern  whales. 


/ 


CHAPTER  XXI 

Geographical  distribution1 — Areas  of  distribution— Barriers  to  migration 
— Means  of  dispersal — Changes  in  the  physical  conditions  of  the  earth's 
surface — The  evidence  afforded  by  the  study  of  geographical  distribution 
with  regard  to  the  theory  of  organic  evolution. 

IT  is  hardly  necessary  to  remind  the  reader  that  each  species 
of  plant  or  animal,  in  a  state  of  nature,  is  more  or  less  sharply 
restricted  to  a  certain  portion  of  the  earth's  surface,  the  entire 
region  over  which  it  may  be  found,  whether  sea  or  land,  being 
termed  its  area  of  distribution.  Such  areas  of  specific  distribu- 
tion are  nearly  always  continuous,  without  any  considerable  gaps 
or  intervals  from  which  the  species  is  entirely  absent.  This  does 
not,  of  course,  mean  that  the  species  necessarily  occurs  in  all 
parts  of  its  area  of  distribution  at  once,  but  that  it  is  free  to 
range  over  the  whole  of  it  and  may  accordingly  be  found  in  any 
suitable  part  of  it  at  any  time.  It  is  necessary  to  introduce  some 
such  qualifying  word  as  "  suitable  "  in  this  connection,  because 
each  species  is  not  only  restricted  in  its  range  to  a  more  or  less 
well-defined  geographical  area,  but  can  only  live  continuously  in 
certain  portions  of  that  area,  to  the  special  conditions  of  which  it 
is  structurally  and  physiologically  adapted  and  which  constitute 
its  habitat.  Thus,  for  example,  a  fresh-water  snail  may  perhaps 
range  over  an  entire  continent,  but  it  would  be  useless  to  look  for 
it  except  in  fresh  water.  Individuals  of  a  species  may  pass  with 
more  or  less  freedom,  according  to  the  nature  of  the  case,  from 
one  habitat  to  another  within  the  area  of  distribution,  but  it  is 
.only  on  rare  and  exceptional  occasions  that  they  are  able  to 
transgress  the  boundaries  of  the  area  itself. 

True  discontinuity  in  areas  of  specific  distribution,  as  distin- 
guished from  mere  discontinuity  of  habitats,  is  extremely  rare. 
We  have  a  good  example  of  it,  however,  in  the  case  of  the  marsh 

1  The  reader  is  referred  to  Dr.  Wallace's  classical  volumes  on  "  Island  Life  "  and 
the  "Geographical  Distribution  of  Animals,"  and  to  Professor  Heilprin's  work  on 
the  "  Distribution  of  Animals"  in  the  International  Scientific  Series  (Vol.  LVIII, 
1887),  for  further  information  on  this  subject. 


320       OUTLINES   OF  E  VOLUTION  ABY  BIOLOGY 

tit  (Parus  palustris),  which  has  two  areas  of  distribution  separated 
from  one  another  by  an  interval  of  four  thousand  miles — in 
Europe  and  Asia  Minor  on  the  one  hand  and  in  Northern  China 
on  the  other. 

The  size  of  the  area  over  which  a  species  may  range  varies 
immensely,  in  some  cases  comprising  an  entire  continent,  or  even 
more,  and  in  others  only  a  few  square  miles.  Thus  the  leopard 
ranges  over  the  whole  of  Africa  and  most  of  Southern  Asia,  while 
the  Tuatara  (Fig.  113)  is  confined  to  certain  small  islands  off  the 
coast  of  New  Zealandr  and  certain  species  of  humming  birds 
are  said  to  occur  only  on  the  volcanic  peak  of  Chimborazo  in  the 
equatorial  Andes  * 

An  area  of  generic  distribution  is  the  sum  of  the  areas  of 
distribution  of  all  the  species  which  are  comprised  within  the 
genus,  and  thus  genera  have  usually  a  much  wider  geographical 
range  than  species.  Families,  again,  have  a  wider  range  than 
genera  and  orders  than  families,  and  so  on  with  groups  of  still 
higher  value.  In  short,  the  more  comprehensive  the  group  the 
larger  will  be  its  area  of  distribution,  until  we  find  that  the  sub- 
kingdoms  or  phyla  are  cosmopolitan,  ranging  more  or  less  over 
the  entire  world,  wherever  a  suitable  habitat  is  to  be  found. 

The  reason  why  species  are  rarely,  if  ever,  cosmopolitan  in 
their  distribution  is  that  they  are  confined  within  their  own 
limited  areas  by  the  existence  of  physical  conditions  which  con- 
stitute what  are  called  barriers  to  migration.  Such  barriers 
either  form  absolutely  insuperable  obstacles  to  the  passage  of  the 
species  in  question  or  they  may  be  surmounted  only  at  rare 
intervals  and  by  some  happy  chance. 

The  nature  of  the  barriers  varies,  of  course,  with  the  species 
concerned,  and  what  is  a  barrier  to  one  may  be  a  high  road 
to  another.  In  the  case  of  marine  animals  the  principal  barriers 
are  continents  and  temperature  conditions,  while  the  deep  sea 
itself  acts  as  a  barrier  to  the  distribution  of  shore-dwelling  or 
littoral  forms.  For  terrestrial  animals  the  chief  barriers  are 
seas,  rivers,  mountain  ranges,  deserts  and  climate  generally ;  for 
fresh-water  animals  land  and  sea.  In  the  case  of  plants  the 
barriers  to  migration  are  very  much  the  same. 

It  may  be  laid  down  as  a  general  law  that  every  organism, 

whether   animal   or  vegetable,  at   some  period  or  other  of  its 

/    existence  is  specially  adapted  so  as  to  secure  dispersal,  either  by 

its  own  exertions  or  by  the  action  of  some  external  agency.     By 


DISPERSAL   OF   PLANTS  321 

sonic  means  or  another  it  is  able,  not  only  to  spread  itself  over 
its  own  area  of  distribution,  but  also,  when  occasion  offers,  to 
extend  that  area  by  surmounting  its  barriers. 

The  lower  terrestrial  plants,  such  as  fungi,  mosses  and  ferns, 
are  dispersed  by  means  of  spores,  which,  protected  by  special 
envelopes,  may  be  widely  distributed  by  the  wind.  In  the 
higher  plants  the  spores,  as  agents  of  dispersal,  are  replaced  by 
seeds,  which,  usually  still  within  the  fruit, maybe  carried  on  the 
wind,  floated  on  rivers  or  ocean  currents,  carried  about  entangled 
in  the  hair  or  feathers  of  animals,  or  actually  eaten  and  passed 
out  uninjured  in  the  faeces.  A  great  many  seeds  and  fruits  are 
specially  modified  in  structure  to  secure  their  distribution  in  one 
or  other  of  these  ways,  and  the  study  of  such  adaptations  consti- 
tutes one  of  the  most  interesting  chapters  in  botanical  science. 
We  need  only  refer  here  to  such  fruits  as  the  blackberry,  whose 
succulence  tempts  the  birds  to  eat  them  and  carry  the  seeds, 
safely  enclosed  in  their  hard  protective  envelopes,  to  long 
distances ;  the  various  kinds  of  burs  with  their  hooks  for  entangle- 
ment in  fur  and  feathers ;  the  winged  fruits  of  the  maple,  elm 
and  ash,  and  the  thistledown  of  the  thistles,  adapted  for  floating 
on  the  wind. 

The  dispersal  of  plants  is  in  all  cases  passive  and  dependent 
on  external  agencies,  though  sometimes  aided  by  some  purely 
mechanical  device  in  the  plant  itself ;  in  the  case  of  animals  it 
may  take  place  either  passively  or  actively,  by  the  exertions  of, 
the  animals  themselves. 

Beginning  with  the  marine  fauna,  we  find  that  the  larger 
forms — whales,  porpoises,  dolphins  and  fishes — owe  their  dis- 
persal mainly  to  their  own  active  powers  of  locomotion,  while 
the  smaller  animals,  especially  the  invertebrates,  are  largely 
dependent  in  this  respect  upon  oceanic  currents. 

Even  animals  which,  like  the  sponges  and  corals,  are  firmly 
fixed  to  the  sea-bottom  in  the  adult  condition,  have  free-swim- 
ming larval  forms  (Fig.  164)  whose  own  limited  powers  of 
locomotion  may,  under  favourable  circumstances,  be  enormously 
supplemented  by  the  action  of  currents.  Such  larval,  forms, 
from  the  point  of  view  of  dispersal,  play  the  part  of  the  spores  and 
seeds  of  plants,  and  they  occur  not  only  in  cases  where  the  adult 
is  strictly  sedentary  in  habit  but  also  where  its  powers  of  loco- 
motion are  limited,  as  in  many  worms,  snails,  crabs  (Fig.  128) 
star-fishes,  brittle  stars  (Fig.  127)  and  sea-urchins.  The 


322        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


dispersal  even  of  large  and  active  fish,  like  the  mackerel,  is  largely 
assisted  by  the  action  of  currents  upon  the  floating  eggs,  and 
this  factor  must  be  of  still  greater  importance  in  the  case  of 
the  comparatively  sluggish  bottom  dwellers,  such  as  the  turbot 
and  sole. 

The  more  superficial  waters  of  the  open  ocean  are  densely 
populated  with  pelagic  animals  and  plants  and  with  pelagic  eggs 
and  larvae  in  various  stages  of  development,  all  drifting  more  or 
less  helplessly  wherever  the  ocean  currents  may  carry  them,  for 
their  own  powers  of  locomotion  are  usually  quite  insufficient  to 

enable  them  to  pursue  an  inde- 
pendent course.  This  floating 
population  is  technically  spoken 
of  as  *'  plankton  "  and  its  in- 
vestigation, which  is  of  great 
importance  for  the  solution  of 
practical  fishery  problems,  has 
lately  attracted  a  great  deal  of 
attention. 

We  must  therefore  regard  all 
the  great  ocean  currents,  such 
as  the  Gulf  Stream,  as  highways 
thronged  with  life  of  many  kinds, 
including  representatives  of  all 
the  more  important  groups  of 
marine  animals,  any  one  of 
which  may  be.  on  its  way  to 
found  a  new  colony  and  establish  its  own  particular  species  in  some 
region  far  distant  from  its  original  home.  Some  of  the  wanderers 
are  only  immature  forms,  belonging  partly  to  shore-dwelling 
species,  others  are  adult.  Almost  all  exhibit  some  special  adap- 
tation to  their  pelagic  mode  of  life.  The  fish  eggs  are  floated 
by  oil -globules,  and  the  larvae  of  the  crabs,  brittle  stars  and 
sea-urchins  are  provided  with  defensive  spines  (Figs.  127,  128) ; 
but  the  most  general  and  characteristic  feature  of  all  the  pelagic 
host  is  transparency,  whereby  they  are  rendered  inconspicuous 
and  less  likely  to  become  the  victims  of  the  numerous  enemies 
which  feed  upon  the  plankton.  Adult  jelly-fish,  worms,  molluscs 
and  crustaceans,  and  innumerable  larval  forms,  all  exhibit  this 
same  peculiarity. 

The  effect  of  ocean  currents  upon  the  distribution  of  marine 


FIG.  164. — Free-swimming  Larva  of 
a  Sponge,  Orantia  compressa ; 
highly  magnified. 

(The  larva  swims  by  means  of  the  rapid 
undulations  of  the  numerous  flagella 
with  which  it  is  provided.) 


DISPEESAL   OF   ANIMALS  323 

animals  is  shown  in  a  very  interesting  manner  in  the  case  of  the 
Mediterranean  and  the  Ked  Sea.  In  each  of  these  there  is  a 
surface  current  constantly  flowing  in  from  the  open  ocean  and 
bringing  in  vast  numbers  of  individuals,  both  larval  and  adult, 
which  never  find  their  way  out  again.  Hence  these  almost 
enclosed  seas  form  a  kind  of  trap  for  marine  animals  and  we 
accordingly  find  them  to  be  inhabited  by  an  exceptionally  rich 
and  varied  fauna. 

The  range  of  marine  species,  though  sometimes  very  wide,  is 
usually  more  or  less  strictly  limited,  so  that  the  shores  of  every 
continent  have  their  own  characteristic  fauna  and  flora.  This  is 
no  doubt  partly  accounted  for  by  differences  in  climatic  conditions, 
food  supply  and  so  forth,  but  it  is  mainly  due  to  the  fact  that, 
in  spite  of  the  facilities  for  travel  afforded  by  ocean  currents, 
the  dangers  incidental  to  a  long  voyage  from  one  continent  to 
another  are  rarely  surmounted,  at  any  rate  by  shore-dwelling 
organisms.  We  know  that  many  such  forms  flourish  quite  as  well 
in  some  other  part  of  the  world  as  in  their  original  home  if  they 
can  once  overcome  the  initial  difficulty  of  migration.  Thus 
the  artificial  introduction  of  the  American  oyster  into  British 
seas  has  accidentally  brought  with  it  the  introduction  of  the 
remarkable  limpet-like  Crepidula,  which  attaches  itself  to  the 
oyster  shells  and  runs  riot  over  the  oyster  beds  on  the  Essex 
coast.  That  such  occurrences  may  occasionally  take  place  in  a 
state  of  nature,  and  a  species  thereby  be  enabled  to  extend  its 
area  of  distribution,  there  can  be  no  reasonable  doubt,  for  even 
American  turtles  have  occasionally  been  carried  by  the  Gulf 
Stream  to  the  shores  of  Great  Britain. 

Amongst  the  higher  forms  of  non-aquatic  animals,  and 
especially  birds  and  mammals,  their  own  powers  of  locomotion 
constitute  the  most  important  means  of  dispersal.  Even  these f 
however,  are  frequently  transported  for  long  distances  by  those 
external  agencies  which  are  chiefly  responsible  for  the  dispersal 
of  less  highly  organized  forms. 

Leaving  out  of  account,  for  the  moment,  the  action  of  man, 
which  has  brought  about  immense  changes  in  the  geographical 
distribution  of  the  existing  fauna,  the  chief  agents  to  be  noticed 
in  this  connection  are  wind  and  water. 

It  is  to  the  action  of  the  wind  that  large  numbers  of  winged 
animals — insects,  birds  and  bats — owe  the  wide  distribution  which 
they  enjo}^.  All-  actively  flying  land  animals  are  liable  to  be 

Y  2 


324        OUTLINES   OF   E VOLUTION AEY  BIOLOGY 

carried  out  to  sea  in  storms,  and  although  the  great  majority  of 
these  will  inevitably  perish  a  few  will  occasionally  manage  to 
reach  some  distant  haven  where  they  may  succeed  in  establish- 
ing a  colony  and  thus  extending  the  range  of  the  species. 

During  westerly  winds  American  birds  not  infrequently  make 
their  appearance  on  various  parts  of  the  coast  of  Europe,  while 
north  of  the  58th  parallel  of  latitude  the  polar  winds  trend  in 
the  opposite  direction  and  with  them  we  find  a  transference  of 
European  birds,  by  way  of  Iceland  and  Greenland,  to  the 
American  continent.1  During  storms,  again,  European  birds  are 
cast  upon  the  Azores,  about  1,000  miles  from  the  nearest 
continental  coast,  and  there  is  strong  reason  for  believing  that 
the  little  wax-eye  (Zoster ops  lateralis)  has  been  transported  in 
this  way  from  Australia  to  New  Zealand,  where  it  has  succeeded 
in  establishing  itself. 

Water  currents  may  play  an  important  part  in  the  dispersal 
of  two  groups  of  terrestrial  animals — those  which  occasionally 
swim  and  those  which  are  liable  to  be  carried  away  on  icebergs 
or  on  floating  masses  of  vegetation.  Most  quadrupeds  swim  well 
and  even  if  not  habitual  swimmers  may  be  forced  to  take  to  the 
water  in  times  of  flood.  In  this  way  they  may  cross  large 
rivers  and  even  get  carried  out  to  sea  and  perhaps  to  some 
neighbouring  island,  but  they  cannot  cross  large  stretches 
of  open  ocean,  and  are  accordingly  never  found,  except 
when  introduced  by  man,  on  islands  far  remote  from  any 
continent. 

In  polar  regions  the  floating  ice  affords  a  means  of  dispersal 
to  such  animals  as  wolves  and  polar  bears,  while  within  the 
tropics  floating  islands  or  rafts  formed  of  matted  vegetation  play 
the  same  part.  Such  islands  have  been  observed  floating  out  to 
sea  from  the  mouths  of  large  rivers  like  the  Ganges,  the  Amazon, 
the  Congo  and  the  Orinoco.  They  serve  as  a  means  of  transport 
to  many  different  kinds  of  terrestrial  reptiles,  birds  and  mammals, 
and  countless  molluscs,  worms  and  insects,  to  say  nothing  of 
plants. 

"  If,"  says  Sir  Charles  Lyell,  "  the  surface  of  the  deep  be 
calm,  and  the  rafts  are  carried  along  by  a  current,  or  wafted  by 
some  slight  breath  of  air  fanning  the  foliage  of  the  green  trees, 
it  may  arrive,  after  a  passage  of  several  weeks,  at  the^bay  of  an 
island  into  which  its  plants  and  animals  may  be  poured  out  as 

1  Heilprin,  op.  cit.,  p.  47. 


DISPEKSAL   OF   ANIMALS  325 

from  an  ark,  and  thus  a  colony  of  several  hundred  new  species 
may  at  once  be  naturalized."  1 

As  a  definite  example  of  this  kind  of  dispersal  may  be  men- 
tioned the  fact  that  in  1827  a  large  boa  constrictor,  twisted  round 
the  trunk  of  a  tree,  was  carried  by  ocean  currents  from  South 
America  to  the  Island  of  St.  Vincent,  where  it  was  destroyed 
after  killing  a  few  sheep. 

A  current  flows  from  the  North  Island  of  New  Zealand 
southwards  to  Chatham  Island,  four  hundred  miles  distant 
from  the  nearest  point  on  the  New  Zealand  coast.  This  current 
carries  considerable  quantities  of  New  Zealand  timber  to  the 
island  and  its  existence  probably  accounts  for  the  fact  that  the 
planarian  worm,  Geoplana  exulans,  has  been  found  both  in  the 
North  Island  of  New  Zealand  and  on  Chatham  Island,  but,  as 
yet,  nowhere  else.  The  land  planarians  habitually  creep  into 
the  crevices  of  decayed  timber,  and  their  eggs  are  enclosed  in 
tough,  horny  cocoons  which  may  probably  occasionally  be 
transported  even  over  wide  stretches  of  sea. 

Small  terrestrial  animals  are,  of  course,  often  accidentally 
dispersed  by  human  agency.  Eats,  mice  and  cockroaches  have 
been  carried  nearly  all  over  the  world  by  ships,  and  snails, 
worms  and  other  small  creatures  may  be  carried  about  with 
timber  and  earth,  especially  around  the  roots  of  plants.  When 
I  was  in  New  Zealand  I  had  some  plants  sent  to  me  from 
England  in  a  tightly  closed  tin  box.  When  they  arrived,  after 
a  voyage  of  some  five  or  six  weeks,  I  found  an  earthworm  still 
alive  in  the  tin.  Many  invertebrates  have  doubtless  been 
unknowingly  dispersed  in  this  manner  and  great  care  has  to  be 
taken  to  make  due  allowance  for  such  possibilities  in  studying 
problems  of  distribution.  In  exactly  the  same  sort  of  way  the 
seeds  of  many  plants  are  accidentally  dispersed  over  the  world 
in  ships'  ballast,  so  that  the  same  common  European  weeds 
occur  in  the  neighbourhood  of  the  ports  along  all  the  great 
routes  of  commerce. 

The  restrictions  placed  upon  the  dispersal  of  fresh  water 
animals  are  more  severe  than  in  the  case  of  either  marine  or 
terrestrial  forms.  One  river  system  or  one  lake  is  separated 
from  another  by  intervening  land  or  sea  which  fresh  water 
animals  cannot  as  a  rule  cross  by  their  own  exertions.  There 
are  of  course  exceptions,  as  in  the  case  of  the  lampreys  and  eels, 

1  Lyell's  "  Principles  of  Geology,"  Ed.  5,  Vol.  III.,  p.  44. 


326        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

and  other  fish  which  go  down  to  the  sea  periodically,  but  for  the 
most  part  the  inhabitants  of  fresh  water  are  largely  dependent 
upon  external  agencies  for  their  dispersal.  Accordingly  we  find 
two  groups  of  such  animals,  widely  contrasted  with  one  another 
as  regards  their  distribution.  Those  which  do  not  go  down  to 
the  sea  and  which  are  not  likely  to  be  carried  about  by  external 
agencies,  such  as  most  of  the  fishes,  have  usually  restricted  areas 
of  specific  distribution,  and  individual  mountain  lakes  sometimes 
contain  peculiar  species  of  fish  which  are  found  nowhere  else  in 


FIG.  165. 


-;>*, 


PIG.  166. 


FIG.,  165.—  Gemmule  or  Statoblast  of  a  Fresh.  Water  Polyzoon,  Cristatella 

mucedo,  X  40.     (From  Sollas.) 

"FiG.  166.  —  Jwo  Gtemmules  of  a  Fresh  Water  Sponge,  Ephydatia  (Spongilla] 
fluviatiCis,  X  60.     (FrorQf  a  photograph.) 

gem.)  gemmules  ;  sp.,  spicules  of  the  parent  sponge. 

the  world.  Galaxias  nigothoruk,  for  example,  is  a  small  fish 
which  occurs  abundantly  in  lake  Nigothoruk  in  Victoria 
(Australia).  This  lake  is  in  a  very  isolated  position  in  a 
mountainous  region  and  the  only  outlet  is  by  percolation  under- 
ground. There  appears  to  be  no  natural  means  by  which  the 
fish  could  be  transferred  to  any  other  locality  at  the  present 
time,  and  it  is  not  known  to  occur  elsewhere. 

On  the  other  hand  many  fresh  water  invertebrates,  such  as 
the  Polyzoa,  hydras  and  sponges,  and  above  all  the  microscopic 
Protozoa,  are  remarkable  for  their  wide  distribution.  Identical 
genera  if  not  identical  species  of  these  groups  occur  almost  all 
over  the  world,  and  the  reason  for  this  is  not  far  to  seek,  for  all 


CLIMATIC    CHANGES  327 

of  them  have  some  special  character  which  enables  them  to  be 
easily  dispersed  by  external  agencies.  The  fresh  water  Polyzoa 
and  sponges  produce  minute  buds  (statoblasts  or  gemmules) 
enclosed  in  hard  protective  envelopes  (Figs.  165,  166),  which  are 
likely  to  be  carried  about  in  the  mud  on  the  feet  of  wading  birds 
and  mammals.  The  embryo  of  Hydra  secretes  its  own  protective 
envelope  (Fig.  59,  D — G)  within  which  it  passes  through  a  period 
of  rest  embedded  in  the  mud  ;  while  many  of  the  Protista  (e.g. 
Haematococcus)  are  capable  of  being  dried  up  at  some  period  or 
other  of  their  life-history  and  carried  about  by  the  wind  in  the 
form  of  dust.  Thus  a  sample  of  mud,  taken  from  a  pond  and 
dried  up,  may,  after  an  interval  of  many  months,  if  again  placed 
in  water,  give  rise  to  an  abundant  fauna,  amongst  which  even 
such  highly  organized  forms  as  Crustacea  (e.g.  Apus  and 
Branchipns,  which  lay  specially  protected  eggs)  will  frequently 
appear. 

We  must  remember  that  the  present  distribution  of  animals 
and  plants  is  the  outcome  not  only  of  the  existing  physical 
conditions  of  the  earth's  surface  but  also  of  conditions  which 
obtained  in  past  geological  periods.  From  time  to  time  these 
conditions  undergo  great  changes,  which  may  concern  not  only 
the  climate  of  particular  regions  or  of  the  entire  world,  but  also 
the  relative  distribution  of  land  and  sea. 

The  earth  has  been  subject,  at  various  periods  of  its  history, 
to  climatic  changes  of  two  chief  kinds,  (1)  cold  or  even  glacial 
epochs  in  temperate  regions,  and  (2)  mild  or  warm  epochs  in 
arctic  or  antarctic  regions.  Probably  alternations  of  these  two 
extremes  have  been  not  infrequent,  but  the  case  of  which  we 
have  most  complete  knowledge  occurred  in  the  Pleistocene  period 
and  is  usually  known  as  the  "  Glacial  Epoch  "  par  'excellc /• 

There  is  clear  evidence  that  during  a  portion  of  the  Pleistocene 
period  a  very  large  part  of  the  northern  hemisphere,  which  now 
enjoys  a  temperate  climate,  was  covered  with  perpetual  ice  *• 
snow  and  reduced  to  a  condition  resembling  that  of  Greenland 
at  the  present  time.  Scandinavia  and  the  whole  of  Northern 
Europe  were  buried  beneath  the  ice-sheet,  and  the  same  is  true, 
of  the  northern  part  of  North  America.  \ 

The  glacial  epoch  in  the. north  must  have  driven  the  greater 
number  of  the  northern  plants  and  animals  southwards,  causing 
a  keen  struggle  for  existence  in  which  many  species  were 
exterminated.  Its  influence  was  possibly  intensified  by  the 


9 


OUTLINES   OF  EVOLUTIONARY  BIOLOGY 


t  that  the  glaciation  was  not  continuous  but  alternated  with  a 
.  succession  jrt  warm  periods.  The  southern  hemisphere  also 
experienced  a  glacial  epoch  during  which  warm  and  cold  periods 
alternated,  and  astronomers  hold  that  the  warm  periods  in  one 
hemisphere  coincided  with  cold  ones  in  the  other.  It  has  been 
calculated  that  each  warm  or  cold  period  lasted  for  about 
21,000  years. 

Other  important  changes  in  climate  occurred  long  before  the 
great  glacial  epoch.  Thus  the  fossil  remains  of  a  luxuriant 
vegetation  in  Greenland  and  other  northern  localities  indicate 
the  occurrence  of  a  mild  arctic  climate  in  Miocene  times.  Such 
a  climate  must  have  favoured  migration  between  the  old  and 
new  worlds  by  way  of  what  is  now  Behring  Strait,  which  may 
very  well  have  been  dry  land  at  the  time. 

Owing  to  the  gradual  loss  of  the  earth's  heat  by  radiation  and 
the   consequent   shrinkage   and   crumpling  of  the  solid  crust, 
variations  in  the  level  of  the  land  are  constantly  taking  place. 
Areas  which  are  at  present  separated  by  sea  may  have  been 
/  connected  in  former  times  and  vice  versa,  and  there  can  be  no 
doubt  that   the   distribution   of   plants  and  animals  has  been 
'  profoundly  influenced  in  this  way.     Many  cases  oJLjjiscontmuity 
i  in  distributiqn  may  be  explained  by  the  former  existence  of  land 
corrections  which  no  longer  remain.     It  is  necessary,  however, 
t"  be  extremely  careful  how  we  invoke  the  aid  of  this  principle, 
v/hich,  as  an  easy  way  out  of  difficulties,  is  apt  to  lead  us  into 
all  sorts  of  unjustifiable  speculations. 

Those  remarkable  animals  the  lemurs,  as  a  group,  exhibit  a 
very  curious  Discontinuity,  in  their  distribution,  occurring  in 
Africa  (and  especially^Madagascar)  on  the  one  hand  and  in 
Southern  Asia  on  the  other.  To  explainjbis^  distribution  it  has 
been  suggested  that  in  former  times  a  continent  —  Lemuria  — 
existed  in  the  Indian  Ocean.  Similarly,  but  with  perhaps 
greater  justification,  it  is  believed  by  many  people  that  the 
antarctic  continent  at  one  time  extended  much  further  north 
than  at  the  present  day,  so  as  to  afford,  possibly  with  the  aid  of 
a  chain  of  islands  and  with  the  co-operation  of  a  mild  antarctic 
climate,  a  route  along  which  migration  might  take  place  between 
South  America  and  Australasia.  In  this  way  may  be  explained 
certain  remarkable  points  of  /agreement  between  the  fauna  and 
flora  of  Australia  and  New^Zealand  and  those  of  South  America. 
The  genus  Fuchsia,  for  example,  is  typically  South  American, 


CHANGES   IN   LAND   AND    SEA  329 

but  one  or  two  species  occur  in  New  Zealand ;  and  the  same  is 
true  of  Calceolaria.  The  evergreen  beech  forests  of  New  Zealand 
must  be  extraordinarily  like  those  which  Darwin  described  in 
Patagonia.  Even  the  same  curious  genus  of  fungus  (Cyttaria) 
is  found  on  the  beech  trees  in  South  America,  New  Zealand  and 
Tasmania.  A  fresh  water  lamprey,  Geotria,  also  occurs  both  in 
New  Zealand  and  South  America  and  similar  cases  could  be 
quoted  from  the  invertebrate  fauna. 

It  is  very  doubtful,  however,  whether  such  an  extensive  change 
in  the  configuration  of  the  earth's  surface  as  the  submergence  of 
an  entire  continent  has  ever  taken  place.  According  to 
Dr.  Wallace,  who  is  recognized  as  the  greatest  authority  on 
the  subject  of  geographical  distribution,  the  existing  continents 
and  oceans  as  a  whole  are  permanent  features,  although  their 
outlines  may  be  greatly  affected  by  oscillations  of  the  earth's 
crust. 

Perhaps  the  strongest  argument  against  the  former  existence 
of  continents  where  we  now  have  oceans  lies  in  the  frc.fc  t^at  the 
average  depth  of  the  sea  is  many  times  greater  than   • 
height  of  the  land,  no  less  than  twelve  thousand  fe 
pared  with  one  thousand,  for  the  great   depths  of  i 
extend  over  vast  areas  while  the  greatest  heights 
are  narrow  mountain  ranges.     Hence,  although  large 
land  might  be  submerged  by  a  comparatively  slight  f. 
level,  it  would  take  an  enormous  movement  to  bring  any 
tract  of  the  ocean  bed  to  the  surface. 

In  short,  we  are  only  justified    in   postulating 
existence  of  land  in  places  where  the  ocean  is  comp 
shallow,  but  even  this  limitation  leaves  abundant  c 
for  changes  in  the  relative  distribution  of  land  and 
would  profoundly  affect  the  distribution  of  plants  ard 
The  actual  occurrence  of  such  changes  is  abundantly 
geological  evidence  and  they  are  known  to  be  going^  o 
pi^eht  day  in  many  parts  of  the  world. 

There  is  good  reason  to  believe  that  the  principal  groups  of 
terrestrial  animals  originated  in  the  great  northern  land  masses 
and  that  the  southern  peninsular  areas  of  Africa,  Australasia 
(now,  of  course,  represented  by  detached  islands)  and  South 
America  have  been  peopled  mainly  by  successive  waves  of 
migration  from  the  north.  We  find  in  all  these  southern  areas 
primitive,  ancient  forms  of  life.  Marsupials  at  the  present  day 


330        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

are  found  only  in  Australasia  and  America,  but  the  fossil  remains 
of  such  animals  are  widely  distributed  over  the  northern  hemi- 
sphere. The  Onychophora,  again,  a  small  group  of  extremely 
primitive  arthropods,  which  until  recently  were  all  included  by 
zoologists  in  the  single  genus  Peripatus  (Fig.  167),  are  almost 
confined  to  Australasia,  South  Africa  and  South  America,  in  all 
of  which  regions  they  are  fairly  abundant.  It  is  more  reasonable 
to  imagine  that  the  ancestors  of  the  Onychophora  migrated 
from  the  north,  where  the  group  has  now  become  extinct, 

than  to  invent  imaginary  continents 
across  which  they  may  have  wandered, 
or  even  to  suppose  that  they  have 
been  so  widely  distributed  as  we  now 
find  them  by  some  external  agency 
such  as  floating  timber. 
ji  The  geographical  distribution  of 
plants  and  animals  would  be  quite 
inexplicable  on  the  supposition  that 
they  had  all  been  independently 
created  and  deposited  where  they 
now  live.  It  is,  however,  easy  enough 
to  explain  it  on  the  theory  that  the 
earth  has  been  peopled  by  the  des- 
'  cendants  of  common  ancestors  which 
migrated  from  place  to  place  as 
FIG.  167.— Peripatus capensis,  occasion  permitted  and  at  the  same 
CProi^^lioto^rpliO  !'  time  underwent  modification  in  many 
different  directions.  We  may  now 

briefly  summarize  the  principal  facts  of  distribution  which  justify 
us  in  holding  this  view. 

*(1)  The  extent  of  the  area  of  distribution  of  any  group  of 
animals  is  directly  proportional  to  its  means  of  dispersal.  Thus 
flying  animals  are  much  more  widely  distributed  than  quadrupeds. 
Birds  occur  abundantly  on  oceanic  islands,  but  the  only  mamn^ls 
which  occur  there  in  a  state  of  nature  are  bats  and  small  forms 
like  rats  and  mice  which  may  be  carried  on  floating  timber. 
Nevertheless  we  know  that  when  the  larger  mammals  are  trans- 
ported by  man  to  such  localities  they  flourish  exceedingly. 
Many  Protozoa,  again,  which  are  readily  blown  about  in  the  form 
of  dust,  are  almost  cosmopolitan  even  as  regards  their  species. 
.  (2)  The  degree  of  peculiarity  of  the  fauna  and  flora  of  any 


GEOGEAPHICAL  ISOLATION  331 

area  is  proportional  to  the  length  of  time  for  which  and  the 
extent  to  which  that  area  has  been  isolated  from  other  areas. 
Thus  Australia,  which  has  probably  been  separated  from  the 
Asiatic  continent  ever  since  the  Cretaceous  period,  has  a  most 
peculiar  fauna  and  flora.  We  have  already  referred  to  the 
numerous  different  kinds  of  marsupials — kangaroos,  wombats, 
phalangers,  native  bears,  native  cats  and  so  forth — which  have 
not  as  yet  been  supplanted  by  the  more  recently  developed 
groups  of  mammals  found  in  other  parts  of  the  world.  Australia 
is  also  still  the  home  of  those  most  primitive  and  reptile-like 
of  all  the  mammals,  the  Monotremata  (Figs.  91,  92).  The 
Australasian  forests,  again,  are  composed  principally  of  eucalypts 
of  many  different  species,  which  are  found  nowhere  else  in  the 
world.  In  New  Zealand,  which  is  even  more  isolated  than 
Australia,  we  find  no  less  peculiar  inhabitants,  including  the 
wonderful  tuatara  (Fig.  113),  the  oldest  surviving  type  of  terrestrial 
vertebrate,  together  with  the  kiwi  (Fig.  Ill)  and  other  remarkable 
flightless  birds. 

The  reasons  why  the  degree  of  peculiarity  of  the  fauna  and 
flora  of  any  region  is  proportional  to  the  degree  of  geographical 
solation  are  not  difficult  to  find.     On  the  one  hand  ancient  types,' 
such  as  the  tuatara,  the  monotremes  and  the  marsupials,  may  be 
preserved  from  competition  with  more  modern  forms  long  after 
they  have  been  exterminated  elsewhere.    On  the  other  hand, indivi- 
duals accidentally  introduced  from  distant  areas  at  rare  intervals 
will  have  few  opportunities  of  breeding  with  others  of  the  same 
species,  and  thus  whatever  variation  occurs  amongst  them  will  S 
be  less  liable  to  be  swamped  by  intercrossing  with  the  parent 
form.     New  races  and  ultimately  new  species  will  thus  become 
established  more  readily  in  such  areas  than  elsewhere.     This>^, 
principle  of  geographical  isolation  as  a  factor  in  the  production  *r 
of  new  species  is  of  great  importance  and  we  shall  have  to  refer   / 
to  it  again  in  a  subsequent  chapter. 

f  he  zoological  or  botanical  affinities  of  the  inhabitants  of  any 
given  area,  not  only  with  one  another  but  also  with  those  of 
adjacent  areas,  are  exactly  what  we  should  expect  in  accordance 
with  the  views  which  we  are  advocating.  It  is  impossible  to 
believe  that  the  existing  marsupials  were  (with  the  exception  of 
the  few  American  species)  all  specially  created  in  Australasia 
when  we  know  perfectly  well  that  marsupials  used  to  exist  in 
Europe  in  past  geological  times  and  can  still  exist  in  Europe 


382        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

when  transported  there  by  human  agency,  and  it  is  equally 
impossible  to  believe  that  such  animals  as  sheep  and  rabbits,  to 
which  the  Australian  climate  appears  to  be  pre-eminently  suited, 
were  specially  created  in  Europe  and  Asia  but  never  in  Australia. 
The  existing  condition  of  the  Australian  fauna  is,  however,  easily 
explained  on  the  supposition  that  it  was  originally  derived  from 
Asia  at  the  time  when  marsupials  and  monotremes  flourished  in 
the  north,  and  that  the  island  continent  became  separated  from 
the  mainland  before  the  more  recent  mammalian  types,  such  as 
sheep  and  rabbits,  had  arisen  on  the  latter.  Divergent  evolution 
within  the  limits  of  this  isolated  area  is  then  quite  sufficient  to 
account  for  the  immense  variety  of  marsupials  occurring  there  at 
the  present  day. 

It  is  very  instructive  in  this  connection  to  contrast  the  con- 
dition of  the  fauna  of  a  comparatively  recently  separated 
continental  island,  such  as  Great  Britain,  which  is  not  far 
removed  from  its  parent  continent,  with  that  of  the  fauna  of  a 
typical  oceanic  island  which  has  never  formed  part  of  a  continent 
at  all  and  is  very  widely  separated  from  any  other  land.  The 
native  or  indigenous  population  of  continental  islands  always 
exhibits  a  close  relationship  with  that  of  the  adjacent  mainland, 
from  which  it  was  originally  derived  and  with  which  it  is  still 
able  to  keep  up  a  certain  amount  of  intercourse.  Such  an  island 
will  contain  indigenous  quadrupeds,  and  the  great  majority  of  the 
species  of  plants  and  animals  found  in  it  will  be  identical  with 
those  of  the  mainland.  True  oceanic  islands,  on  the  other  hand, 
such  as  St.  Helena  and  the  Sandwich  Islands,  are  peopled 
entirely  by  waifs  and  strays  which  have  gained  access  to  them  at 
rare  intervals  in  one  or  other  of  the  ways  discussed  in  the  earlier 
part  of  this  chapter.  They  never  contain  large  quadrupeds  and, 
owing  to  their  more  or  less  complete  isolation,  the  animals  which 
do  occur  almost  always  belong  to  peculiar  species  found  nowhere 
else  in  the  world. 

|^  (3)  Palaeontological  investigations  have  demonstrated  that  the 
present  animal  population  of  any  tolerably  isolated  area  is  closely 
related  to  the  population  of  the  same  area  in  comparatively 
recent  geological  periods.  Thus  in  Australia  we  not  only  find 
that  at  the  present  day  marsupials  are  by  far  the  most  charac- 
teristic features  of  the  fauna,  but  also  that  the  remains  of  extinct 
marsupials,  many  of  which  .belong  to  genera  and  species  different 
from  any  now  living,  are  very  abundant  in  the  tertiary  deposits 


DISCONTINUOUS   DISTRIBUTION 

of  the  same  region.  Similarly  in  South  America  at  the  present 
time  the  edentates  (sloths,  armadillos  and  ant-eaters)  form  the 
most  characteristic  mammalian  group,  and  the  tertiary  deposits 
of  that  country  have  yielded  the  remains  of  a  great  number  of 
extinct  forms  belonging  to  the  same  order.  It  would  be  very 
difficult  to  explain  these  facts  on  any  theory  of  special  creation, 
but  we  can  easily  understand  how  a  group  of  animals,  having 
once  gained  a  footing  in  any  area  and  finding  itself  secure  and 
more  or  less  cut  off  from  communication  with  other  parts  of  the 
world,  would  increase  and  vary,  producing  new  species  and 
ultimately  becoming  the  dominant  group  in  that  particular 
region. 

/  (4)  Cases  of  discontinuous  distribution  are  readily  explicable 
on  the  theory  of  evolution  and  migration.  Either  individuals 
of  the  species  in  question  have  occasionally  transgressed  the 
barriers  to  their  dispersal  and  established  new  and  distant 
colonies,  or  possibly  a  large  area  of  distribution  has  become 
broken  up  into  a  number  of  smaller  ones  by  geographical  or 
climatic  changes  rendering  portions  of  it  uninhabitable. 

"  Thus,  for  instance,"  says  Eomanes,  "  it  is  easy  to  understand 
that  during  the  last  cold  epoch  the  mountain  hare  would  have 
had  a  continuous  range ;  but  that  as  the  arctic  climate  gradually 
receded  to  polar  regions,  the  species  would  be  able  to  survive  in 
southern  latitudes  only  on  mountain  ranges,  and  thus  would 
become  broken  up  into  many  discontinuous  patches,  correspond- 
ing with  these  ranges.  In  the  same  way  we  can  explain  the 
occurrence  of  arctic  vegetation  on  the  Alps  and  Pyrenees — 
namely,  as  left  behind  by  the  retreat  of  the  arctic  climate  at  the 
close  of  the  glacial  period."  1 

1  "  Darwin  and  after  Darwin,"  Vol.  I.,  p.  209. 


CHAPTER  XXII 

Adaptation  to  environment  in  animals — Deep  sea  animals — The  colouration 
of  animals — Protective  and  aggressive  resemblances — Warning  colours 
— Mimicry — Epigamic  ornamentation. 

IN  the  last  few  chapters  we  have  discussed  a  number  of  facts 
selected  from  that  great  and  ever  increasing  mass  of  evidence 
which  leads  us  to  the  inevitable  conclusion  that  the  present  con- 
dition of  the  fauna  and  flora  of  the  earth,  with  their  almost 
endless  diversity  of  plants  and  animals,  is  the  outcome  of  a  long 
process  of  organic  evolution.  It  is  desirable  at  this  stage  of  our 
inquiry  to  emphasize  the  fact  that  this  evolution,  in  the  main, 
has  been  of  a  progressive  character,  and  of  such  a  character, 
moreover,  as  to  maintain  a  more  or  less  perfect  harmony  between 
the  organism  and  its  environment. 

Adaptation  in  bodily  organization  and  in  corresponding 
function,  whereby  each  kind  of  plant  or  animal  is  enabled  to 
meet  the  constant  demands  made  upon  it  and  maintain  its 
existence  in  the  endless  warfare  of  life,  is  the  great  outstanding 
feature  of  living  things.  So  universal  is  this  adaptation  that 
we  are  apt  to  take  it  for  granted,  and  any  want  of  it  is  at  once 
recognized  as  an  exception  and  an  anomaly.  Anyone,  for 
example,  who  watches  the  slow  and  clumsy  movements  of  a 
tortoise  cannot  fail  to  be  struck  with  the  fact  that  the  limbs  of 
this  animal  are  but  ill-suited  for  purposes  of  locomotion,  but 
even  in  this  case  there  is  compensation  in  that  the  tortoise 
carries  its  place  of  refuge  about  with  it  and  has  therefore  little 
need  to  hurry  itself. 

We  have  seen  in  an  earlier  chapter  how  completely  the 
pentadactyl  limbs  of  air-breathing  vertebrates  may  become 
modified  from  their  primitive  condition  in  correspondence  with 
changes  in  the  mode  of  life.  The  fore  limbs,  adapted  in  the 
first  instance  for  locomotion  on  land,  have  become  changed  in 
the  whales,  seals  and  dugongs  into  paddles ;  in  the  pterodactyls, 
birds  and  bats  into  wings,  and  in  man  into  organs  of  prehension. 


DEEP   SEA   ANIMALS 


335 


Indeed,  given  time  enough,  the  power  which  an  organism 
possesses  of  altering  its  bodily  structure  in  accordance  with  new 
demands  on  the  part  of  the  environment  seems,  as  we  have 
already  pointed  out,  to  be  almost  without  limits. 

This  plasticity  is  illustrated  in  the  most  striking  manner  in 
cases  where  the  organism  has  been  removed  from  what  may  be 
regarded  as  the  normal  environment  of  the  group  to  which  it 
belongs,  and  to  which  the  great  majority  of  the  group  are  adapted, 
and  come  to  live  under  new 
and  very  different  conditions. 
Thus  it  is  with  the  aquatic 
and  aerial  mammals,  which, 
in  encroaching  upon  the 
domains  of  the  fishes  and 
birds,  have,  by  convergent 
evolution,  come  to  resemble 
these  in  bodily  form. 

Wherever  we  turn  we  find 
fresh  illustrations  of  the  same 
principle.  At  great  depths 
of  the  ocean  the  conditions  of 
life  are  very  different  from 
those  which  obtain  in  shal- 
low water,  and  we  find  the 
animals  which  inhabit  these 
abysses  modified  accordingly. 
Fig.  168  represents  two  deep 
sea  sponges  obtained  by  the 
"  Challenger  "  expedition  ; 
Cladorhiza  longipinna  from  a 
depth  of  3000  fathoms  in  the  North  Pacific  and  Axoniderma  mirabile 
from  a  depth  of  2250  fathoms  in  the  South  Pacific.  It  will  be  seen 
at  once  that  the  form  assumed  by  these  sponges!  s  very  unusual 
and  quite  unlike  that  exhibited  by  their  shallow  water  relatives. 
The  great  majority  of  the  members  of  the  group  of  sponges  (the 
Tetraxonida)  to  which  they  belong  are  indeed  by  no  means 
remarkable  for  symmetry  of  shape,  but  these  two  are  beautifully 
symmetrical,  their  form  at  once  suggesting  that  of  a  parachute, 
with  a  small  conical  body  fringed  by  long  radiating  processes 
surrounding  a  central  root-like  projection.  This  "  Crinorhiza 
form,"  as  it  is  termed,  is  obviously  an  adaptation  which  serves 


FIG.  168.  —  Two  Deep  Sea  Sponges, 
exhibiting  the  Crinorhiza  Form. 
A.  Cladorhiza  longipinna;  B,  Axoni- 
derma mirabile;  nat.  size.  (After 
Eidley  and  Dendy  in  "  Challenger  " 
Eeports.) 


336        OUTLINES   OF   EYOLUTIONAKY  BIOLOGY 

to  prevent  the  sponge  from  sinking  into  and  being  smothered  by 
the  soft  mud  or  ooze  which  covers  the  bottom  of  the  ocean  at 
very  great  depths,  and  it  is  interesting  to  observe  that  species  of 
several  distinct  though  related  genera  have  adopted  the  same 
device,  thus  affording  a  beautiful  example  of  the  phenomenon  of 
convergence.  Other  sessile  deep  sea  animals  have  found  different 
means  of  overcoming  the  same  difficulty,  especially  in  many 
cases  by  the  development  of  long  stalks. 

The  absence  of  light  at  great  ocean  depths  has  led  to  the 
acquisition  on  the  part  of  many  of  the  deep  sea  fishes  of  brilliant 
phosphorescent  organs,  arranged  like  little  lamps  on  various 
parts  of  the  body.  In  some  cases  at  any  rate  these  serve  to 
attract  other  animals  upon  which  these  fishes  prey.  Some  of 
them,,  again,  develop  long  and  delicate  feelers  by  aid  of  which 
they  grope  their  way  about  in  the  dark. 

In  the  brilliantly  illuminated  surface  waters  of  the  ocean  con- 
ditions are  very  different,  and  here  we  find  that  the  most  favourite 
device  for  preserving  life  amidst  a  host  of  enemies  is  transparency, 
but  we  have  already  alluded  to  this  in  the  preceding  chapter  and 
need  not  dwell  upon  it  further.  It  is  a  phenomenon  which  falls 
under  the  head  of  protective  colouration,  of  which  we  shall  find 
better  instances  elsewhere. 

.  The  significance  of  the  colouration  of  animals  as  a  means  of 
adaptation  to  environment  is  a  subject  which  has  in  recent  years 
developed  into  a  special  branch  of  biological  science,  and  which 
already  has  a  copious  literature  of  its  own.  Professor  Poulton, 
in  his  well  known  work  on  the  Colours  of  Animals,1  has  suggested 
an  elaborate  scheme  of  colour  classification  from  this  point  of 
view.  He  distinguishes,  in  the  first  place,  between  apatetic 
(deceitful)  colours,  sematic  (warning  and  signalling)  colours,  and 
epigamic  colours  (displayed  in  courtship),  all  of  which  afford 
marvellous  instances  of  more  or  less  highly  specialized  adaptation. 
We  have  not  space  to  follow  out  the  details  of  this  classification 
but  we  shall  presently  refer  to  examples  of  all  the  more  important 
types  of  colouration  included  therein. 

Every  observer  of  nature  must  have  been  struck  with  the 
general  harmony  of  colouration  which  exists  between  animals 
and  their  surroundings.  So  complete  is  this  harmony  that  our 
sense  of  hearing  is  frequently  a  better  guide  to  the  whereabouts 
of  an  insect,  bird  or  mammal  than  our  sense  of  sight.  I 

'  International  Scientific  Series,  Vol.  LXV1II. 


PROTECTIVE   AND  AGGRESSIVE    RESEMBLANCE   337 


remember  standing  with  my  gun  in  the  midst  of  a  dense  patch 
of  scrub  in  Australia  and  hearing  the  pademelons1  hopping  about 
all  around  me.  For  a  long  time,  however,  I  could  see  nothing 
but  the  trees.  My  native  guide  pointed  out  where  I  was  to  aim, 
but  I  only  fired  at  a  log  from  the  side  of  which  a  pademelon 
hopped  away.  Again  he  pointed,  and  this  time  at  a  small  white 
spot  which  I  could  just  distinguish  amongst  the  trees.  I  fired 
once  more,  aiming  at  the  white  spot,  and  sure  enough  a  pademelon 
rolled  over.  It  appeared  that  I  had  aimed  at  the  white  fur  which 
occurs  on  the  breast  of  the  animal  and  which  to  the  experienced 
eye  of  the  native  told  all  that  he  needed  to 
know.  It  is  often  supposed  that  conspicuous 
patches  of  this  kind  serve  as  recognition  marks 
between  individuals  of  the  same  species,  but  it 
may  be  questioned  how  far  the  advantage  of 
being  recognized  by  a  friend  compensates  for  a 
disturbance  of  the  colour  harmony  which  reveals 
an  animal  to  its  enemies. 

The  type  of  colouration  which  aids  in  the 
concealment  of  an  animal  is  termed,  by  Pro- 
fessor Poulton,  cryptic.  It  belongs,  of  course, 
to  the  apatetic  group.  Concealment  may  be 
desirable  either  as  a  means  of  escape  from 
enemies  or  for  the  purpose  of  ambuscading 
prey,  or  possibly  for  both.  In  the  former  case 
we  may  speak  of  it  as  protective  resemblance 
(procryptic  colouration),  in  the  latter  as  aggres- 
sive resemblance  (anticryptic  colouration). 

Protective  resemblance  is  often  of  a  very  highly 
specialized  character,  and  may  be  due  as  much  to  adaptation 
in  actual  form  as  to  adaptation  in  colour ;  frequently  these  two 
factors  unite  in  producing  the  result,  and  a  third  may  be  added, 
viz.,  adaptation  in  habit  or  instinct.  In  the  common  stick 
caterpillars  of  the  geometer  moths  we  see  all  three  factors 
co-operating.  In  colour  and  shape  these  caterpillars  precisely 
resemble  small  twigs.  They  move  about  with  a  characteristic 
looping  action,  amongst  the  leaves  or  branches  of  the  bushes 
which  they  frequent,  but  when  at  rest  they  stiffen  themselves  up 
and  stand  out  from  the  branch  at  the  exact  angle  of  a  twig,  and 


FIG.  169. — Larva 
of  the  Brim- 
stone Moth 
(Rumia  cratae- 
gata)  resting 
upon  a  Haw- 
thorn twig  ; 
nat.size.  (From 
Poulton.) 


1  A  small  species  of  kangaroo. 


338        OUTLINES   OF   E VOLUTION AEY  BIOLOGY 


in  this  condition  it  is  extremely  difficult  to  detect  them.     Professor 
Poulton  remarks : — 

"These  caterpillars  are  extremely  common,  and  between  two  and 
three  hundred  species  are  found  in  this  country ;  hut  the  great 
majority  are  rarely  seen  because  of  their  perfect  resemblance  to 
the  twigs  of  the  plants  upon  which  they  feed." 

As  will  be  seen  from  the  illustration  (Fig.  169),  which  represents 
the  larva  of  the  brimstone  moth  upon  its  food  plant,  the  hawthorn, 

the  caterpillar  is  enabled  to  main- 
tain its  position  for  a  long  period 
by  attaching  its  head  to  a  twig  by 
means  of  a  silken  thread. 

Numerous  moths  so  closely  re- 
semble in  the  colour  and  pattern 
of  the  upper  surface  of  their  wings 
the  objects  upon  which  they  rest 
in  the  daytime,  such  as  the  bark 
of  trees,  that  they  are  almost 
invisible,  but  perhaps  the  most 
perfect  examples  of  protective 
resemblance  are  met  with  in  the 
wonderful  leaf  insects.  Fig.  170 
represents  an  orthopterous  insect, 
Pulcliripliylliiim  crurifolium,  from 
Ceylon.  The  whole  insect  is  of  a 
bright  leaf-green  colour,  and  not 
only  are  the  wings  shaped  and 
veined  so  as  to  resemble  leaves, 
but  even  the  body  and  legs  exhibit 
leaf -like  outgrowths. 

In  the  well  known  Indian  leaf 
butterfly,  Kallima  (Fig.  171),  the  resemblance  to  a  leaf  is  only 
seen  when  the  insect  comes  to  rest  with  its  wings  folded  together 
above  the  body  so  as  to  expose  their  under  surfaces.  It  is  a  dry, 
dead  leaf  which  is  imitated  this  time,  and  stalk  and  midrib,  veins 
and  colour  markings,  even  down  to  such  minutiae  as  rust  spots, 
are  perfectly  represented. 

The  Mantidae  or  praying  insects  feed  upon  flies,  &c.,  which 
they  capture  with  marvellous  dexterity  with  their  serrated  claws. 
In  some  species  the  uniform  green  colouration  doubtless  serves, 
not  only  to  protect  them  from  their  own  enemies,  but  also  to 


FIG.  170.  A  Green  Leaf  Insect 
(Pulchripfiyllium  crurifolium, 
?  ),  from  Ceylon;  X  £.  (From 
a  photograph.) 


PROTECTIVE    AND  AGGRESSIVE   RESEMBLANCE    339 

prevent  them  from  being  seen  by  their  victims  before  they  have 
come  within  range.  Other  species  exhibit  even  more  wonderful 
adaptations  both  in  form  and  colour.  Thus  the  South  African 
Harpax  tricolor  &its  amongst  the  pink  and  white  flowers  of  the 
heath,  which  are  imitated  by  similarly  coloured  outgrowths  of 
the  insect,  and  there  awaits  the  approach  of  its  unsuspecting 
victims;  while  in  Mozambique  the  terrible  Idolum  diabolicum 


FIG.  171. — An  Indian  Leaf  Butterfly  (Kallima   inacliis};   A.,  with  wings 
expanded  ;  B.,  with  wings  folded  ;    X  f .     (From  a  photograph.) 

simulates,  both  in  form  and  colour,  a  large  flower,  and  thereby 
deceives  and  attracts  other  insects  in  search  of  honey. 

It  is  no  doubt  amongst  the  almost  innumerable  species  of  the 
great  group  Insecta  that  cases  of  highly  specialized  adaptation 
for  purposes  of  concealment  or  deception  are  most  frequently 
met  with.  They  also  occur,  however,  and  by  no  means 
•uncommonly,  in  other  groups  of  the  animal  kingdom.  A 
familiar  instance  is  afforded  by  the  common  British  spider  crab, 
now  known  as  Macropodia  rostrata,1  of  which  excellent  illus- 
trations (under  the  name  Cancer  Phalangium)  were  given  by 

1  1  am  indebted  to  my  friend,  the  Rev.  T.  R.  R.  Stebbing,  P.R.S.,  for  information 
as  to  the  correct  nomenclature,  &c.,  of  this  species. 

z  2 


340        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Dr.  Macculloch,  in  the  Transactions  of  the  Linnean  Society,  as 
far  back  as  1801.  I  am  enabled  by  the  courtesy  of  the  Council 
of  the  Society  to  reproduce  here,  on  a  reduced  scale,  Dr.  Maccul- 
loch's  original  plate  (Fig.  172).  The  crab  actually  breaks  off 
fronds  of  seaweed  and  attaches  them  to  the  long  hairs  of  its  body, 
thus  disguising  itself  so  effectually  as  to  be  quite  unrecognizable 
except  by  careful  examination.  Dr.  Macculloch  was  of  opinion 


I 


FIG.  172. — Reduced  Facsimile  of  Dr.  Macculloch's  Plate  of  Macropodia 
rostrata,  in  tlie  Transactions  of  the  Linnean  Society.  On  the  left  is 
shown  a  plant  of  the  seaweed  in  which  the  crab  dresses  itself  up ;  on 
the  right  the  crab  without  the  seaweed,  and  at  the  bottom,  the  crab 
dressed  up. 

that  this  dressing  up  of  the  crab  in  seaweed  was  an  artifice 
which  assisted  it  in  capturing  its  food  (anticryptic) ,  but  it  is 
much  more  likely  that  it  is  protective  (procryptic).  The  late 
Professor  Bell  has  told  us  how  the  slow  and  sluggish  habits  ol 
the  crab  render  it  an  easy  prey  to  fishes,  and  the  stomach  of  a 
thornback  ray  has  been  found  entirely  filled  with  them,  so  that 
there  appears  to  be  ample  reason  for  them  to  seek  concealment. 

In  the  case  of  Macropodia  the  adaptation  for  concealment 
shows  itself  as  an  inherited  habit  or  instinct  more  than  in  any 
modification  of  bodily  structure,  but  such  an  instinct  is  probably 


PKOTECTIVE   AND   AGGRESSIVE   RESEMBLANCE    341 

itself  the  effect  of  some  structural  modification,  however  impossible 
to  detect,  in  nervous  tissue.  In  the  Australian  Phyllopteryx 
eques,  a  fish  which  is  closely  related  to  the  curious  sea-horse 
(Hippocampus)  of  our  own  coasts,  we  get  precisely  the  same 
idea,  so  to  speak,  carried  out  in  a  different  manner.  Both 
Hippocampus  and  Phyllopteryx  live  amongst  seaweed,  to  which 
they  attach  themselves  by  means  of  their  curious  prehensile 
tails.  Hippocampus  (Fig.  173)  exhibits  no  special  resemblance 


FIG.  174. 


FIG.  173. 

FIG.    173. — A  Sea-horse   (Hippocampus  antiquorum},    X    f.     (From   a 

photograph.) 

FIG.  174. — Phyllopteryx  eques,  attached  to  seaweed.     (From  Giinther's 
"Study  of  Fishes.") 

to  its  surroundings,  but  in  Phyllopteryx  (Fig.  174)  the  body  is 
covered  with  cutaneous  outgrowths  which  float  out  in  the  water 
like  fronds  of  seaweed  and  doubtless  effect  a  most  satisfactory 
disguise.  This  is  certainly  a  less  troublesome  plan  than  that  of 
dressing  up  in  clothing  borrowed  from  the  outside  world. 

The  well  known  colour  changes  of  the  chamaeleon  and  of 
various  flat  fishes,  not  to  mention  numerous  other  instances 
which  might  be  cited  from  different  groups  of  the  animal  king- 
dom, are  due  to  a  complex  apparatus,  controlled  by  the  nervous 


342        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

system,  whose  function  it  is  to  bring  about  a  varying  adaptation 
for  concealment  under  varying  conditions  of  the  environment. 
How  perfect  the  adaptation  may  be  will  be  realized  by  all  who 
have  ever  observed  with  what  marvellous  accuracy  the  colour 
markings  of  a  turbot  in  an  aquarium  are  made  to  match  the 
sand  or  gravel  upon  which  it  is  lying. 

In  striking  contrast  with  the  cryptic  colouration  by  which  an 
animal  seeks,  as  it  were,  to  avoid  observation,  are  those  numerous 
cases  in  which  self-advertisement  appears  to  be  the  main  object 
in  view.  The  British  army,  which  only  in  recent  yeajES  has 
learnt  the  advantages  of  khaki  clothing  when  in  the  field,  still 
exhibits  some  of  the  most  startling  instances  of  conspicuous 
colouration  met  with  anywhere  in  the  animal  kingdom,  though 
whether  these  examples  should  be  classed  under  the  head  of 
warning  colours,  or  regarded  as  belonging  to  the  epigamic 
category,  is  perhaps  an  open  question.  We  must,  however,  con- 
fine our  attention  in  this  place  to  a  few  examples  of  warning 
colours  met  with  amongst  the  lower  animals* 

We  have  seen  that  both  warning  and  signalling  colours,  or 
recognition  marks,  are  spoken  of  as  sematic.  The  former  are 
further  distinguished  as  aposematic  and  the  latter  as  episematic. 
Aposematic  colours  are  exhibited  by  many  animals  which  possess 
some  special  means  of  defence  and  find  it  advantageous  to 
advertize  the  fact.  Wasps  and  hornets,  with  their  conspicuous 
orange-  and  black-banded  bodies,  are  excellent  examples.  Such 
animals  do  not  seek  to  conceal  themselves  but  rely  upon  their 
warning  colours  to  remind  their  enemies  that  they  had  better 
leave  them  alone.  It  is  not  enough  that  they  should  possess  the 
power  of  making  themselves  disagreeable;  the  fact  must  be 
clearly  recognized,  otherwise  they  would  be  constantly  exposed 
to  experimental  attack,  and  suffer  injuries  for  which  any  damage 
which  they  might  inflict  upon  their  pursuers  would  be  but  a 
poor  consolation.  Orange,  red  and  black,  owing  to  their  great 
conspicuousness,  especially  when  associated  with  one  another, 
are  the  colours  most  frequently  met  with  in  this  connection,  and 
we  find  these  colours,  not  only  in  noxious  insects,  but  in  various 
vertebrates,  such  as  certain  poisonous  reptiles,  toads  and  sala- 
manders. The  Gila  monster  (Heloderma  suspectum),  of  Mexico 
and  Arizona,  is  the  only  known  poisonous  lizard,  and  is  con- 
spicuously coloured  in  tints  of  blackish  brown,  yellow  and 
orange,  while  other  members  of  the  same  group  are  usually 


MIMICRY 


343 


coloured  so  as  to  harmonize  more  or  less  perfectly  with  their 
surroundings. 

If  it  is  advantageous  for  a  noxious  species  to  advertize  its  true 
character,  it  is  no  less  so  for  an  innocuous  one  to  advertize  a 
false  character,  and  gain  credit  for  some  power  of  making  itself 
objectionable  which  it  does  not  really  possess.  The  practice  of 
bluffing  is  by  no  means  an  exclusively  human  institution.  Thus 
we  find  many  insects,  which  in  themselves  are  quite  inoffensive, 
taking  on  the  characteristic  warning  colouration  of  dangerous 
species.  The  drone-fly  mimics  the  bee,  and  though  they  belong 
to  widely  different  orders  of  insects, 
the  one  having  only  two  wings  and  the 
other  four,  the  resemblance  is  so  close 
as  to  have  given  rise,  as  we  saw  in  an 
earlier  chapter,  to  the  ancient  belief 
in  the  spontaneous  generation  of  bees 
from  the  carcases  of  oxen  (on  which,  of 
course,  drone-flies  had  deposited  their 
eggs). 

Most  moths,  as  is  well  known,  have 
opaque  wings,  covered  with  microscopic 
scales,  but  in  the  clear-winged  moths 
(Fig.  175,  A)  the  wings  have  partially 
lost  their  scales  and  become  transparent, 
and  this  anomalous  feature,  combined 
with  the  colouration  of  the  body,  enables 
these  perfectly  harmless  insects  to  mimic 
the  dangerous  hornets  (Fig.  175,  B). 
Even  a  harmless  snake  may  mimic 
the  warning  colouration  of  a  venomous 
species,  and  thus  secure  for  itself  the  respect  which  is  properly 
due  only  to  the  latter. 

It  is  not  necessary  that  an  animal  should  be  capable  of 
inflicting  serious  injury  upon  its  enemies  when  attacked  for  it  to 
secure  immunity  from  pursuit  as  soon  as  recognized.  Many 
butterflies  and  other  insects,  which  are  probably  merely  distasteful 
or  nauseous  (or  perhaps  actually  unwholesome)  to  birds,  exhibit 
aposematic  or  warning  colouration.  Amongst  these  we  find 
curious  associations  known  as  synaposematic  groups,  the 
members  of  which,  belonging  to  distinct  species  and  often  by 
no  means  closely  related  to  one  another,  seem  to  have  combined 


B. 


FlG.  175.  —  A.,  a  clear- 
winged  Moth  (Sesia  cra- 
broniformis)  mimicking 
B.,  a  .Hornet  (Vespa 
crdbro] ;  both  X  f .  (From 
a  photograph.) 


344        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

together  to  share  the  expenses  of  a  common  advertisement  and 
thereby  reduce  the  cost  to  each.  ^J£pung  birds  have  to  learn 
by  experience  which  insects  are  goJeFto  eat  and  which  are  not. 
In  making  their  experiments  no  doubt  they  themselves  suffer, 
but  the  subjects  of  the  experiment  are  probably  actually  killed. 
Obviously,  then,  if  one  experiment  can  be  made  to  serve  for  a 
number  of  different  species  of  insects  there  will  be  a  corre- 
sponding reduction  in  the  death-rate,  and  hence  it  is  that  we 


FIG.  176. — A  Synaposematic  Group  of  South  American  Lepidoptera,  all  x  -J. 
(From  a  photograph.) 

A,  Tithorea  liarmonia  ;  B,  Heliconius  ethilla;  C,  Perrhybris  (Mylothris)  malenka. 
g  ;  D,  Dismorphia  praxinoe,  J  ;  E,  Pericopis  angulosa. 

find  these  groups  of  species  all  adopting  the  same  type  of 
warning  colour,  and  thus  coming  to  resemble  one  another  very 
closely,  although  perhaps  belonging  to  totally  distinct  families. 

We  may  illustrate  this  somewhat  complex  phenomenon  by 
reference  to  certain  South  American  Lepidoptera  which  take 
part  in  the  formation  of  such  a  synaposematic  group.  In 
Fig.  176  A,  B  and  D  represent  butterflies  belonging  to  three 
distinct  families,  while  E  is  a  moth,  as  may  be  seen  at  once  by 
its  thick  body  and  the  absence  of  terminal  knobs  on  the  antenna}. 


SYNAPOSEMATIC   GROUPS  345 

All  of  these,  in  common  with  numerous  other  species  which 
inhabit  the  same  area,  Ij^ve  adopted  the  same  characteristic 
scheme  of  warning  colouration,  wherein  the  prevailing  tints  are 
black  and  orange. 

In  such  a  synaposematic  group,  or  mimicry  ring,  it  is  usually 
possible  to  distinguish  between  certain  species  which  seem  to 
have  led  the  way  in  the  development  of  the  warning  colouration, 
and  others  which  seem  to  have  followed  their  example.  In 
the  particular  case  under  notice  the  original  "  models  "  belong 
to  the  group  Ithomiinae,  of  which  Tithorea  harmonia  (Fig.  176,  A) 
is  a  representative.  These  are  probably  the  most  distasteful 
members  of  the  combination  to  birds.  They  have  been  imitated 
by  Heliconinae,  such  as  Heliconius  ethilla  (Fig.  176,  B),  Pierinae 
("whites"),  such  as  Dismorphia  praxinoe  (Fig.  176,  D),  and 
Hypsidae  (a  family  of  moths),  as  exemplified  by  Pericopis  angulosa 
(Fig.  176,  E),  all  of  which  may  be  regarded  as  mimics  of  the 
Ithomiinae. 

The  case  of  the  pierine  mimics  is  particularly  instructive, 
and  shows  very  clearly  that  these  forms  really  imitate  other 
species,  for  the  female  is  commonly  a  far  more  perfect  mimic 
than  the  male,  which  often  departs  little,  if  at  all,  from  the  typical 
colouration  of  the  group  to  which  it  belongs.  Fig.  176,  C  repre- 
sents a  male  pierine,  Perrhybris  (Mylothris)  malenka,  which  is 
at  once  recognizable  from  its  colouration  as  a  "  white,"  although 
even  here,  curiously  enough,  there  is  a  faint  trace  of  the  warning 
colouration  on  the  under  surface  of  the  hind  wings.1  The 
female  of  the  same  species  has  the  warning  colouration  well 
developed,  as  it  is  in  both  male  and  female  of  Dismorphia 
praxinoe. 

So  different  are  the  males  and  females  of  some  of  these 
mimicking  species  that  it  would  be  difficult  to  believe,  were  it  not 
for  breeding  experiments,  that  they  are  really  specifically 
identical.  The  explanation  of  the  difference  is  doubtless  to  be 
found  in  the  fact  that  it  is  much  more  important,  from  the  point 
of  view  of  the  species,  that  the  females,  heavily  laden  with  the 
eggs  upon  which  the  existence  of  future  generations  depends, 
should  be  able  to  warn  off  the  birds,  than  that  the  males  should 
do  so,  for  the  latter,  having  once  accomplished  the  fertilization 
of  the  eggs,  is  of  no  further  value  to  the  race. 

1  Doubtless  inherited  incompletely  from  female  ancestors,  as  in  the  case  of  the 
vestigial  nipples  of  man. 


346        OUTLINES   OF   E VOLUTION ABY  BIOLOGY 

When  an  unquestionably  harmless  species  mimics  the  warning 
colours  of  an  undoubtedly  noxious  one,  the  case  is  sometimes 
spoken  of  as  one  of  "  Batesian  "  mimicry,  after  the  distinguished 
naturalist,  H.  W.  Bates,  who  added  so  much  to  our  knowledge  of 
the  subject.  In  the  case  of  a  synaposematic  group,  or  mimicry 
ring,  however,  it  is  often  impossible  to  say  whether  any  particular 
species  is  edible  or  not,  and  it  may  very  well  be  that  in  some 
cases  all  are  more  or  less  inedible,  though  undoubtedly  some, 
which  are  presumably  the  less  objectionable  forms,  mimic  others, 
which  are  presumably  the  more  objectionable.  This  kind  of 
mimicry,  resulting  in  the  development  of  a  warning  colour 
common  to  a  number  of  inedible  species,  is  sometimes  distin- 
guished as  "Miillerian"  mimicry,  after  the  naturalist  Fritz 
Miiller,  who  first  suggested  the  correct  interpretation  of  the 
phenomenon. 

Perhaps  the  most  remarkable  case  of  mimicry  known 
amongst  butterflies  is  that  of  certain  species  of  Papilio  found  in 
Africa  and  Madagascar,  which  have  formed  the  subject  of 
exhaustive  study  by  Trimen,  Poulton  and  others.  In  Madagascar 
occurs  Papilio  meriones,  a  non-mimetic  species  in  which  the 
male  and  female  (Fig.  177,  A)  closely  resemble  one  another  and 
both  possess  the  "  tail  "  on  the  hind  wing  which  is  such  a  charac- 
teristic feature  of  the  genus.  We  may  take  it,  then,  that  this  is  a 
primitive  form.  On  the  continent  of  Africa  is  found  the  wide- 
spread Papilio  dardanus,  with  several  subspecies.  In  these  sub- 
species the  male  (Fig.  177,  BI)  retains  the  ancestral  form,  but  in 
most  of  them  the  female  is  mimetic ;  it  has  lost  the  Papilio  tail 
and  closely  mimics,  both  in  shape  and  colour  markings,  some 
one  or  another  of  various  species  of  butterflies  belonging  to 
different  families  which  occur  in  the  same  region.  Nor  is  this 
all,  for  the  female  is  likewise  polymorphic,  and  different 
individuals  of  the  same  subspecies  resemble  widely  different 
models.  Thus  the  subspecies  merope  is  known  to  have  three 
forms  of  female,  a  hippocoon  form  (Fig.  177,  B2)  which  mimics 
the  danaine  butterfly  Amauris  niavius  (Fig.  177,  C),  a  trophonius 
form  (Fig.  177,  B3)  which  mimics  another  danaine,  Limnas 
chrysippus  (Fig.  177,  D),  and  a  planemoides  form  which  mimics 
the  acraeine,  Planema  poggei.  Our  illustrations,  which  are 
reproduced  from  Mr.  Trimen's  original  memoir,  give  a  good 
idea  of  the  form  and  pattern  of  some  of  these  insects,  but  they 
lack  the  beautiful  colouring  of  the  original  figures,  which  is 


MIMICRY   IN   BUTTERFLIES 


347 


A.  rapilio   irierion.es    $  Bi.  Papilio  dard.an.us  (merope)  o* 


C  .  Ainauri  5    niaviu  s 


s .  Papilio  dardami.s  (merope) 
{h/pjoocoon  form) 


D.   Limnas     chrysippu.5*  ^3.  Papilio  dardanus  (mp.rc^  % 

\frrojofconius  form} 

FIG.  177. — Mimicry  in  Butterflies.     (After  Trimen,  from  coloured  plates  in 
the  Transactions  of  the  Linnean  Society,  First  Series,  Vol.  XXVI.) 


348        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

necessary  in  order  to  give  a   true  idea  of  the  close  resemblance 
between  mimics  and  models. 

Breeding  experiments  carried  out  by  Mr.  G.  F.  Leigh  with  the 
subspecies  cenea  of  Papilio  dardanus  have  shown  that  the  eggs 
laid  by  one  and  the  same  butterfly  may  give  rise  to  at  least 


FIG.  178. — Male  and  Female  of  an  Australian  Lyre-Bird  (Menura  superld). 
(Photographed  from  specimens  in  the  British  Museum,  Natural  History.) 

three  different  forms  of  mimetic  female,  as  well  as,  of  course,  the 
male. 

It  is  obvious  that  we  have  here  the  very  opposite  of  a 
synaposematic  group,  for,  instead  of  concentrating  upon  a  single 
warning  pattern,  different  individuals,  even  of  the  same  species, 
adopt  totally  different  patterns  in  imitation  of  totally  different 
models.  This  case  still  requires  a  great  deal  of  explanation,  but 
concerning  the  facts  there  can  be  no  doubt. 


EPIGAMIC   ORNAMENTATION  349 

Frequenters  of  any  good  museum  of  natural  history  will  be 
familiar  with  plenty  of  examples  of  epigamic  ornamentation. 
The  highly  elaborate  and  gorgeous  plumage  of  many  male  birds, 
such  as  the  peacock,  the  Argus  pheasant,  the  Australian  lyre- 
bird (Fig.  178)  and  numerous  species  of  humming  birds,  to  say 
nothing  of  less  conspicuous  examples,  affords  the  best  illustration 
of  this  phenomenon.  In  all  such  cases  the  function  of  the 
ornamentation  appears  to  be  to  gratify  the  aesthetic  sense  of  the 
female  during  the  period  of  courtship,  and  render  her  amenable 
to  the  attentions  of  her  mate.  There  is  no  doubt  that  in  many 
cases  the  cock  bird  deliberately  displays  himself  to  the  best 
advantage  before  the  admiring  eyes  of  the  hen,  who  is  credited 
with  a  no  less  deliberate  choice  of  the  partner  who  most  nearly 
approaches  her  ideal  standard  of  beauty.  In  all  these  cases  the 
plumage  of  the  female  is  comparatively  sombre  and  uninteresting. 
An  elaborate  and  gaudy  tail  would  be  a  disadvantage  during 
the  lengthy  period  of  incubation,  when  it  is  desirable,  both 
for  her  own  sake  and  that  of  her  offspring,  that  the  female 
should  be  as  inconspicuous  as  possible.  Thus  it  is  the  male  bird 
that  has  had  to  adapt  his  clothing  to  the  requirements  of  the 
female,  and  she  herself  is  unable  to  follow  the  fashions  which  she 
imposes  upon  her  mate. 

It  is  obvious  that  no  theory  of  evolution  can  be  regarded  as 
satisfactory  which  does  not  offer  some  explanation  of  the  origin  of 
such  highly  specialized  and  precise  adaptations  as  those  which  we 
have  been  considering  in  this  chapter,  and  it  was  in  order  to 
emphasize  the  need  for  such  explanation  that  we  have  laid  so 
much  stress  upon  them.  We  shall  see  in  our  next  chapter  that 
no  less  remarkable  and  precise  adaptations  for  special  purposes 
are  also  met  with  in  the  vegetable  kingdom,  and  shall  then  pass 
on  to  seek  the  necessary  explanation. 


CHAPTER  XXIII 

Adaptation  to  the  environment  in  plants — Alpine  plants,  desert  plants  and 
lianes — The  modification  of  flowers  in  relation  to  insect-fertilization. 

IN  plants  no  less  than  in  animals  we  find  adaptation  to  the 
conditions  under  which  they  have  to  live  to  be  the  most  striking 
feature  of  their  organization.  We  have  already  noticed,  in 
dealing  with  the  phenomenon  of  convergent  evolution,  the 
manner  in  which  Alpine  plants  of  many  kinds  tend  to  assume  the 
compact  cushion-like  form  which  seems  best  suited  to  withstand 
the  rigorous  climate  to  which  they  are  exposed.  Wherever  a 
plant  may  be  found  growing  in  a  state  of  nature  the  character  of 
the  environment  is  sure  to  be  reflected  more  or  less  obviously  in 
its  structure  and  habit.  We  see  this  equally  clearly  in  the 
water-storing  plants  of  desert  regions — the  cacti  of  America 
or  the  aloes  of  Africa — with  their  succulent  stems  or  leaves  and 
other  structural  modifications  which  enable  them  to  withstand 
the  effect  of  long-continued  drought,  and  in  the  climbing  lianes 
of  tropical  forests,  whose  one  object  in  life  appears  to  be  to  reach 
some  elevation  where  they  can  expose  their  foliage  to  the  light 
and  air.  Just  as  we  find  those  air-breathing  mammals  which 
have  taken  to  an  aquatic  life  adopting  the  form  and  habits  of 
fishes  as  being  best  suited  to  their  changed  conditions,  so  in  the 
tropical  forests  of  Queensland  we  find  palms,  members  of  a  group 
which  elsewhere  are  types  of  self-supporting  independence, 
adopting  the  form  and  habit  of  climbing  plants  as  the  only  means 
of  coping  with  the  exigencies  of  the  situation. 

If  the  adaptation  amongst  plants  usually  appears  less  remark- 
able than  is  often  the  case  in  animals,  it  is  because  the 
relations  of  a  plant  to  its  environment  are  usually  less  complex 
than  those  of  an  animal.  The  greater  activity  of  animals  is 
associated  with  the  development  of  highly  specialized  organs  of 
locomotion,  sense-organs  and  nervous  system,  all  of  which  are 
alike  subject  to  adaptation.  Plants  afford  much  more  restricted 
opportunities  for  the  effects  of  the  environment  to  show  them- 
selves, and  it  is  in  their  relations  with  animals,  or  to  speak  more 


ADVANTAGE   OF   CROSS-FERTILIZATION          351 

accurately,  in  those  organs  which  are  immediately  concerned 
with  these  relations,  that  adaptation  becomes  most  complex  and 
precise. 

Even  amongst  animals,  however,  it  would  be  difficult  to  find 
better  illustrations  of  accurate  adaptation  to  highly  specialized 
conditions  of  the  environment  than  those  which  we  see  in  the 
wonderful  structural  modifications  whereby  the  majority  of 
flowers  are  adapted  for  pollination  by  insect  agency.  This 
process  of  pollination  is  commonly  referred  to  as  the  "  fertiliza- 
tion "  of  the  flower,  although,  as  we  have  seen  in  a  previous 
chapter,  the  real  act  of  fertilization  takes  place  inside  the 
so-called  ovule  and  consists  in  the  conjugation  of  the  male  and 
female  gametes,  the  sperm-cells  and  egg-cells. 

The  great  majority  of  flowers  produce  both  pollen  and  ovules, 
containing  respectively  the  male  and  female  gametes  ;  in  other 
words  they  are  hermaphrodite.  It  is  obvious  that  in  such  cases 
we  have  two  possibilities  with  regard  to  fertilization,  for  the 
flower  may  either  be  fertilized  by  its  own  pollen  or  by  that  of 
some  other  flower.  It  may  be  either  "  self-fertilized  "  or  "  cross- 
fertilized,"  and  the  cross-fertilization  may  be  effected  either  by 
pollen  from  another  flower  of  the  same  plant  or  by  pollen  from  a 
different  plant  of  the  same  kind,  the  latter  being  the  more 
advantageous. 

It  seems  at  first  sight  a  strange  thing  that  when  a  flower  is 
capable  of  self-fertilization  such  a  roundabout  and  apparently 
unnecessary  proceeding  as  cross-fertilization  should  ever  take 
place  at  all.  It  has  been  shown,  however,  that  cross-fertilization, 
if  not  absolutely  necessary,  is  at  any  rate  of  very  great 
advantage  to  the  plant,  or  rather  to  the  species,  in  which  it 
occurs. 

Charles  Darwin  experimented  for  eleven  years  on  this  subject, 
and  proved  conclusively  that  cross-fertilization  yields  better 
results  than  self-fertilization,  both  as  regards  the  number  of  seeds 
produced  and  also  as  regards  the  quality  of  the  offspring.  It 
would  be  impossible  to  give  an  adequate  account  of  his  work  in 
this  place,  but  we  may  briefly  notice  one  series  of  experiments 
and  refer  for  the  remainder  to  his  classical  volume  on  the  "  Cross 
and  Self  Fertilization  of  Plants."  He  started  with  the  "  Con- 
volvulus major"  (Ipomcea purpurea),  the  flowers  of  which  are 
hermaphrodite  and  may  be  either  cross-  or  self-fertilized  in  a 
state  of  nature.  It  is  an  easy  matter,  by  artificially  conve}d^g 


352        OUTLINES  OF  EVOLUTIONARY  BIOLOGY 

the  pollen  on  the  tip  of  a  feather,  to  bring  about  the  fertilization 
in  any  way  which  may  be  desired.  Ten  flowers  were  thus  self- 
fertilized  with  their  own  pollen,  while  ten  others  were  cross- 
fertilized  with  pollen  from  a  distinct  plant.  The  crossed  and 
self-fertilized  seeds  thus  obtained  were  carefully  cultivated  under 
exactly  the  same  conditions,  and  it  was  found  that  the  plants 
raised  from  cross-fertilized  seeds  were  taller  than  those  raised 
from  self-fertilized  seeds  in  the  proportion  of  100  to  76. 

The  same  experiment  was  repeated  with  ten  successive 
generations  of  the  same  plants,  and  always  with  the  same  result 
in  favour  of  the  cross-fertilized  individuals.  Moreover,  it  was 
proved  at  the  same  time  that  the  fertility  of  the  plants  produced 
by  cross-fertilization  was  greatly  superior  to  that  of  the  self- 
fertilized  plants,  a  much  larger  quantity  of  seed  being  produced. 

These  results,  taken  in  conjunction  with  many  others  of  the 
same  kind,  clearly  proved  the  advantages  of  cross-fertilization. 
To  quote  Darwin's  own  words  : — 

"It  has  been  shown that  the  offspring  from  the 

union  of  two  distinct  individuals,  especially  if  their  progenitors 
have  been  subjected  to  very  different  conditions,  have  an 
immense  advantage  in  height,  weight,  constitutional  vigour,  and 
fertility  over  the  self-fertilized  offspring  from  one  of  the  same 
parents." 

The  theoretical  explanation  of  this  advantage  is  not  an  easy 
matter,  but  is  probably  to  be  sought  in  the  admixture  of  two 
distinct  series  of  hereditary  tendencies  in  the  offspring  of  a 
cross. 

In  a  state  of  nature  it  is  a  very  rare  thing  for  plants  to  be 
exclusively  self -fertilized,  for,  although  self-fertilization  may  take 
place  in  some  cases  to  a  very  large  extent,  and  although  some 
flowers  are  so  constructed  that  self-fertilization  alone  is  possible, 
yet  there  is  nearly  always  at  least  an  occasional  cross  by  the 
introduction  of  pollen  from  a  different  plant. 

Considering  the  advantages  which  arise  from  cross-fertilization^, 
we  need  not  be  surprised  to  find  that  a  very  large  number  of 
flowers  are  provided  with  special  adaptations  whereby  these 
advantages  may  be  secured  to  them.  We  may  notice  first  certain 
contrivances  by  means  of  which  self-fertilization  is  more  or  less 
effectually  prevented  and  the  injurious  effects  of  perpetual  close 
inbreeding  thereby  avoided.  These  contrivances  may  be  classed 
under  three  principal  heads. 


FERTILIZATION  OF   FLOWERS.  853 

(1)  Separation  of  the  Sexes. — In  many  flowers  we  find  that  only 
stamens  or   pistil   are   developed,   never   both,  so   that  we   get 
distinct  male  and  female  flowers,  which  renders  self-fertilization 
absolutely   impossible,  though   it   does  not   necessarily  prevent 

ertilization  by  pollen  from  other  flowers  on  the  same  plant. 

(2)  Physiological  Self-sterility. — In  some  flowers,  to  quote  the 
words  of  Darwin,  "  the  ovules  absolutely  refuse  to  be  fertilized 

)y  pollen  from  the  same  plant,  but  can  be  fertilized  by  pollen 
rom  any  other  individual  of  the  same  species."  Again,  in  a 
arge  number  of  plants,  although  self-fertilization  may  take  place, 
yet,  if  the  pollen  from  another  individual  be  brought  to  the 
stigma,  it  takes  precedence,  so  to  speak,  of  the  flower's  own 
>ollen  and  renders  the  latter  ineffectual ;  it  is  said  to  be 
>repotent.. 

(3)  Dichogamy. — In  a  large  number  of  flowers,  in  which  both 
male  and  female  organs  are   present,  the   stamens   and  pistil 

Become  mature  at  different  times,  so  that  self-fertilization 
cannot  possibly  take  place  and,  physiologically  speaking,  the 
lowers  are  unisexual.  Usually  in  these  cases  the  pollen  ripens 
and  is  all  shed  before  the  stigma  is  ready  to  receive  it.  The 
lower  is  then  termed  protandrous,  as  for  example  in  the 
common  pink  (Dianthus).  More  rarely  the  stigma  ripens  and 
withers  before  the  pollen  is  ripe,  and  such  species  are  termed 
protogynous,  as  in  the  fig-wort  (Scrophularia  nodosa). 

Of  course  it  would  be  worse  than  useless  to  prevent  self- 
'ertilization  unless  some  means  were  provided  at  the  same  time 
tor  securing  cross-fertilization.  We  saw  in  Chapter  VIII  that 
:he  principal  agents  in  conveying  pollen  from  one  flower  to 
another  are  wind  and  insects,  and  that  flowers  are  accordingly  dis- 
Dinguished  as  anemophilous  or  wind-fertilized  and  entomophilous 
or  insect-fertilized.  The  fact  that  the  former  are  usually  very 
small  and  inconspicuous,  as  for  example  in  the  grasses,  while  the 
latter  are  large  and  brightly  coloured,  affords  strong  presumptive 
evidence  that  entomophilous  flowers  have  become  modified  in 
relation  to  the  insects  which  visit  them  in  search  of  food  or 
shelter.  A  great  many  different  kinds  of  insects  have  this  habit, 
bees,  flies,  butterflies,  moths  and  even  beetles;  while  in  South 
America  some  of  the  humming  birds  in  like  manner  play  the 
part  of  pollen  carriers. 

We  are  in  the  habit  of  regarding  bees  as  the  most  important 
insects  concerned  in  the  cross-fertilization  of  flowers,  and  in 

23.  A    A 


854        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


countries  where  they  are  plentiful  this  is  perhaps  the  case. 
They  visit  the  flowers  in  order  to  collect  both  honey  and  pollen, 
to  be  used  as  food  for  themselves  and  their  offspring.  The  honey 
is  usually  found  at  the  bottom  of  a  long  narrow  tube  formed  by 

the  lower  part  of  the  corolla, 
so  that  in  order  to  reach  it 
the  bee  requires  a  corre- 
spondingly long  and  narrow 
instrument.  This  is  pro- 
vided in  the  si  i  ape  of  a  very 
complicated  proboscis  (Fig. 
179),  formed  by  modifica- 
tion, especially  elongation, 
of  certain  of  the  appendages 
surrounding  the  mouth, 
which  in  more  primitive 
insects,  like  the  cockroach, 
remain  short  and  simple. 
When  not  in  use  the  pro- 
boscis is  neatly  folded  away 
beneath  the  head,  but  when 
a  flower  is  visited  it  is  un- 
folded and  inserted  into  the 
tube  containing  the  honey, 
which  is  then  drawn  up  into 
the  bee's  stomach  by  means 


FIG.  179. — Head  of  a  Bee,  showing  thecom- 


of    a    special    sacking    ap- 


plex  Proboscis  formed  from" modified  paratus.  In  butterflies  and 
Mouth  Parts.  (From  Weismann's  moths  also  a  somewhat 
"  Evolution  Theory.") 


at,  antennae;  Au,  large  compound 
ocellus ;  la.,  labrum  or  upper  lip  ; 


3;  au, 
,  outer 

division  of  second  maxilla  (paraglossa) ;  Zi, 
ligule  or  tongue,  formed  by  fusion  of  inner 
divisions  of  second  maxillae ;  md,  mandible ; 
mxl,  mx\  first  and  second  maxillae ;  pi, 
labial  palp;  p:m.,  maxillary  palp'. 


similar  proboscis  is  used  for 
the  same  purpose,  but  it 
differs  so  much  from  that  of 
the  bee  in  details  of  struc- 
ture as  to  indicate  that  it  has 
been  independently  evolved 
from  the  primitive  mouth  parts  of  some  remote  insect  ancestor. 
In  both  cases  the  mouth  appendages  have  become  specially  adapted 
for  the  very  special  purpose  of  sucking  honey,  ai:d  the  necessity 
for  the  fulfilment  of  the  same  function  has  led,  as  usual,  to  a 
superficial  resemblance  between  the  two  types  of  proboscis.  We 
have  here,  of  course,  another  illustration  of  convergent  evolution. 


FERTILIZATION  OF  FLOWERS.  355 

For  collecting  pollen  the  bees  make  use  of  their  legs,  on  which 
special  collecting  brushes  and  baskets  are  formed  by  the  stiff 
hairs.  The  pollen  is  first  moistened  with  honey  to  make  it  stick 
together,  and  then  picked  up  on  the  collecting  brushes,  placed 
in  the  baskets  on  the  hind  legs  and  carried  to  the  hive  or  nest. 

Insects  commonly  visit  a  large  number  of  flowers  of  the  same 
kind  in  rapid  succession,  and,  whether  they  intentionally  collect 
pollen  for  their  own  purposes  or  not,  it  is  evident  that,  after  each 
visit  to  a  flower  containing  ripe  pollen  grains,  they  will  uninten- 
tionally carry  away  some  of  these,  accidentally  attached  to  various 
parts  of  the  body.  The  pollen  which  is  thus  unconsciously 
carried  away  is  equally  unconsciously  deposited  on  the  stigma  of 
the  next  flower  visited,  and  thus  cross-fertilization  is  effected. 
It  may,  of  course,  happen  sometimes  that  the  pollen  is  deposited 
on  the  wrong  kind  of  flower,  but  that  is  of  no  consequence,  for 
pollen  is,  with  rare  exceptions,  quite  incapable  of  fertilizing 
flowers  of  any  but  its  own  particular  species. 

Under  these  circumstances  it  will  obviously  be  advantageous 
to  a  plant  to  make  its  flowers  as  attractive  to  insects  as  possible, 
and,  just  as  it  is  desirable  for  a  nauseous  insect  to  inform  the 
birds  by  means  of  warning  colours  of  the  fact  of  its  unpalatability, 
so  also  is  it  desirable  for  flowers  which  have  honey  to  offer  in 
exchange  for  the  service  of  pollination  to  make  that  fact  known 
to  their  insect  visitors  by  means  of  brilliant  colours  and  strong 
scents.  There  can  be  no  reasonable  doubt  that  the  colours  serve 
to  attract  the  insects  and  to  enable  them  to  recognize  the  flowers 
which  they  prefer,  and  that  the  same  is  true  of  the  scents  is  very 
clearly  indicated  by  the  fact  that  certain  flowers,  such  as  various 
species  of  Arum  and  related  genera,  which  are  fertilized  by 
carrion-loving  flies,  have  managed  to  perfume  themselves  in 
accordance  with  the  tastes  of  their  visitors. 

It  is  also  desirable  to  ensure,  by  means  of  mechanical  arrange- 
ments, that  the  insects  shall  not  get  their  honey  without  paying 
for  it,  that  is  to  say,  without  effecting  fertilization,  and  this  end 
is  usually  secured  by  concealing  the  honey  at  the  bottom  of  a 
long  tube,  in  such  a  manner  that  the  insect  must  brush  against 
the  stamens  and  stigma  before  it  can  reach  it.  Sometimes,  how- 
ever, the  insects  become  a  little  too  clever  for  the  flower  and  steal 
the  honey  by  biting  a  hole  through  the  bottom  of  the  tube  with- 
out ever  touching  the  stamens  or  stigma  at  all.  In  this  way  the 
red  clover  flowers  are  ^ften  robbed  of  their  honey  by  the  humble 

A   A    2 


356        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


bee,  Bombus  terrestris,  thus  affording  an  example  of  imperfect 
adaptation. 

\  We  may  now  consider  a  few  definite  examples  of  the  manner 
in  which  flowers  may  be  specially  adapted  in  structure  so  as  to 
secure  the  advantages  of  cross-fertilization  by  insect  agency. 

If  a  number  of  plants  of  the  common  primrose,  the  oxlip,  the  cow- 
Blip  or  the  polyanthus  (species  of  the  genus  Primula)  be  examined 
carefully,  it  will  be  seen  that  in  each  case  two  quite  different 
forms  of  flower  occur.  In  other  words  the  flower  is  dimorphic. 

The  two  forms  are  known  to 
gardeners  as  "  pin-eyed "  and 
"  thrum-eyed ' '  respectively.  In  the 
pin-eyed  flowers  (Fig.  180,  A)  the 
style  (g)  is  comparatively  long  and 
the  stigma  (n)  appears  as  a  round 
knob,  like  the  head  of  a  pin,  in  the 
centre  of  the  flower,  at  the  entrance 
to  the  long  tube  formed  by  the 
lower  part  of  the  corolla.  The 
anthers  (a)  lie  much  lower  down  in 
the  tube,  so  that  they  are  invisible 
until  the  flower  is  cut  open.  In  the 
thrum-eyed  flowers  (B)the  positions 
of  the  stigma  and  anthers  are 
reversed ;  the  style  being  much 
shorter  the  stigma  lies  only  half 
way  up  the  tube,  while  the  anthers 
appear  in  the  centre  of  the  flower, 

in  the  mouth  of  the  tube.  The  two  kinds  of  flower  are 
always  found  on  separate  plants,  and  the  long-styled  and  short- 
styled  plants  are  said  to  occur  in  about  equal  numbers  under 
natural  conditions.  The  pollen  grains  also  differ  in  the  two 
kinds  of  flower,  those  of  the  thrum-eyed  being  larger  than  those 
of  the  pin-eyed  and  of  somewhat  different  shape.  This  particular 
kind  of  dimorphism,  which  is  sometimes  known  as  heterostylism, 
is  extremely  characteristic  of  the  genus  Primula,  and  it  has 
recently  been  shown  that  the  distinguishing  features  of  the  two 
forms  are  inherited  in  a  Mendelian  fashion. 

The  flowers  of  the  primulas  are  fertilized  by  the  agency  of 
insects  such  as  humble  bees,  and  Darwin  found  that  if  insects 
v>ere  carefully  excluded  by  covering  the  flowers  with  a  net,  little 


A 


\ 


B 


FIG.  180.— -Heterostyled  Flowers 
of  the  Oxlip  (Primula  elatior] 
in  longitudinal  section. 
(From  Vines'  "  Botany.") 

A,  long-styled;  B,  short-styled  flower, 
a,  anthers;  c,  corolla;  f,  ovary; 
g,  style ;  k,  calyx  ;  n,  stigma. 


FERTILIZATION  OF  FLOWEHS.  357 

or  no  seed  was  produced.  In  order  to  reach  the  honey,  which  is 
secreted  at  the  bottom  of  the  long  tube  of  the  corolla,  the  bee 
thrusts  its  proboscis  down  the  tube  and  in  so  doing  brushes  past 
the  stigma  and  the  anthers.  If  a  pin-eyed  flower  be  visited  the 
pollen  from  the  anthers  is  deposited  comparatively  low  down  on 
the  proboscis.  If  a  thrum-eyed  flower  be  visited  the  pollen  is 
deposited  much  higher  up,  in  accordance  with  the  more  elevated 
position  of  the  anthers.  Thus  the  bees  carry  about  two  different 
kinds  of  pollen  on  two  different  parts  of  the  proboscis,  a  fact  which 
has  been  established  by  microscopic  examination  of  the  proboscis 
itself  with  the  attached  pollen. 

If  it  be  remembered  that  in  the  two  different  kinds  of  flower 
the  relative  positions  of  the  anthers  and  stigma  are  reversed,  it 
will  be  obvious  that  when  a  bee  sucks  honey  from  a  long-sCyled 
flower  the  stigma  will  be  touched  and  pollinated  by  that  part  of 
the  proboscis  which  touches  the  anthers  of  a  short-styled  flower, 
and  vice  versd.  Hence  it  follows  that  pin-eyed  flowers  will  be 
fertilized  by  pollen  from  thrum-eyed,  and  thrum -eyed  flowers  by 
pollen  from  pin-eyed. 

Here,  then,  we  have  a  very  precise  adaptation  for  a  special  kind 
of  cross-fertilization,  and  Darwin  further  proved  by  his  experiments 
that  this  is  the  only  kind  of  fertilization  which  results  in  complete 
fertility.  Cross-fertilization  in  these  plants  is,  however,  possible 
in  no  less  than  four  distinct  ways : — (1)  A  pin-eyed  flower  may 
be  fertilized  by  pollen  from  another  pin-eyed  ;  (2)  a  thrum-eyed 
flower  may  be  fertilized  by  pollen  from  another  thrum- eyed  ; 
(3)  a  pin-eyed  flower  may  be  fertilized  by  pollen  from  a  thrum- 
eyed  ;  (4)  a  thrum-eyed  flower  may  be  fertilized  by  pollen  from 
a  pin-eyed.  The  first  two  methods  have  been  termed  by  Darwin 
"illegitimate  unions"  and  they  result  in  incomplete  fertility ;  the 
last  two  have  been  termed  "  legitimate  "  and  result  in  complete 
fertility. 

The  benefit  derived  from  the  existence  of  the  two  kinds  ol 
flower  lies  in  the  intercrossing,  not  merely  of  two  distinct 
flowers,  but  of  two  distinct  plants,  for  it  will  be  remembered  that 
each  plant  bears  only  one  kind  of  flower.  Self-fertilization  is 
not  absolutely  prevented  in  this  case,  for  the  anthers  and  stigma 
are  mature  at  the  same  time  in  the  same  flower,  but  it  is  not 
likely  to  take  place,  and  if  it  does  it  results  in  incomplete,  fertility 
and  weakly  offspring.  But  even  if  pollen  should  accidentally  find 
its  way  to  a  stigma  of  its  own  plant  it  need  not  necessarily 


358       OUTLINES  OF  EVOLUTIONARY  BIOLOGY 


prevent   cross-fertilization,  for  foreign  pollen  will  be  prepotent 
even  if  deposited  on  the  stigma  some  time  afterwards. 

"  To  test  this  belief,"  Darwin  observes,  "  I  placed  on  several 
stigmas  of  a  long-styled  cowslip  plenty  of  pollen  from  the  same 
plant,  and  after  twenty-four  hours  added  some  from  a  short- 
styled  dark-red  polyanthus,  which  is  a  variety  of  the  cowslip. 
From  the  flowers  thus  treated,  thirty  seedlings  were  raised,  and 
all  these,  without  exception,  bore  reddish  flowers,  so  that  the 
effect  of  pollen  from  the  same  form,  though  placed  on  the  stigmas 

twenty-four  hours  pre- 
viously,  was  quite  des- 
troyed by  that  of  pollen 
from  a  plant  belonging  to 
the  other  form." 

The  flowers  of  the 
common  sage,  and  of 
other  species  of  the  genus 
Salvia  (Fig.  181),  afford  a 
no  less  striking  example 
of  profound  structural 
modification  in  adapta- 
tion to  the  visits  of  insects'. 
These  flowers  are  pro- 
FIG.  181. --Flower of Salcia^ratensis.  (From  tandrous,  the  stamens 

"VVoisminnn's   *'    l<!vnliTf.irm    TTiPnrv  "  aff.fir  .  -,         ,        .,  ... 

maturing  and  shedding 
their  pollen  before  the. 
stigma  is  ready  to  receive 
it,  so  that  self-fertiliza-/ 
tion  is  absolutely  prevented.  Moreover,  the  stigma  in  young 
flowers  (Fig.  181,  gr')  lies  in  such  a  position  that  it  will  not 
be  Jouched  by  visiting  insects,  which  crawl  right  under  it  in 
order  to  reach  the  honey  at  the  bottom  of  the  corolla-tube.  In 
older  flowers  the  style  elongates  and  curves  downwards,  so  that 
the  stigma  (Fig.  181,  gr")  comes  to  lie  in  the  mouth  of  the 
corolla-tube  and  must  be  brushed  against  by  insects  in  search  of 
honey. 

The  most  remarkable  feature  of  the  flower,  however,  is  a 
special  mechanical  contrivance  for  placing  the  pollen  on  the  back 
of  any  insect  which  attempts  to  suck  honey  from  it  in  its  young 
or  male  condition.  There  are  only  two  fully  developed  stamens, 
and  these  are  most  curiously  modified  in  structure.  Each  anther 


U 


Weismanu's  "  Evolution  Theory,"  after 
H.  Muller.) 

gr'  immature  stigma;  </;•">  mature  stigma;  st', 
anther-lobe  concealed  in  the  "  helmet "  ;  st", 
anther-lobes  lowered  ;  U",  lower  lip  of  corolla. 


OF  FLOWERS.  B59 

consists  as  usual  of  two  anther-lobes  united  together  by  a  con- 
nective ;  but  the  connective  is  very  greatly  elongated,  so  that  the 
anther- lobes  are  widely  separated  from  one  another  instead  of 
lying  close  together  at  the  top  of  the  filament.  The  anther-lobe 
at  one  end  of  eacli  connective  is  imperfect  and  produces  little  or 
no  pollen ;  the  other  is  fully  developed.  The  connective  is 
attached  to  the  top  of  the  very  short  filament  by  a  movable  joint, 
so  that  it  can  be  swung  freely  up  and  down  in  a  vertical  plane. 
It  has  a  long  limb  which  curves  upwards  inside  the  "helmet "  of 
the  corolla  and  terminates  in  the  perfect  anther-lobe,  and  a 
short  one  which  bears  the  imperfect  lobe,  as  shown  in  the  figure. 
The  two  stamens  lie  side  by  side  in  the  flower  and  the  imperfect 
anther-lobes  are  held  downwards  in  the  mouth  of  the  corolla- 
tube,  exactly  in  the  path  of  a  visiting  insect.  The  perfect  anther- 
lobes,  on  the  other  hand,  are,  at  first,  held  up  well  above  the 
level  of  the  insect's  back  (Fig.  181,  stf). 

When  a  large  insect,  such  as  a  bee,  visits  the  flower,  it  alights 
upon  the  large  lower  lip  ((7),  which,  as  in  so  many  other  flowers, 
affords  a  convenient  platform  and  has  doubtless  been  specially 
adapted  to  that  end.  The  head  of  the  insect  is  then  thrust  into  the 
corolla-tube  and  pushes  against  the  two  imperfect  anther-lobes. 
The  connectives  turn  on  their  pivots  like  a  see-saw  and  the  two 
ripe  anther-lobes  (st")  are  clapped  down  on  the  back  of  the  insect 
and  dust  it  with  pollen.  This  happens  when  the  insect  visits  a 
young  flower.  When  an  older  flower,  in  the  female  condition,  is 
visited,  the  pollen  from  the  back  of  the  insect  is  deposited  on  the 
mature  stigma  (gr"),  which  now  hangs  directly  in  front  of  the 
entrance  to  the  corolla-tube. 

It  is,  however,  in  the  highly  specialized  order  Orchidaceae  that 
we  find  perhaps  the  most  remarkable  contrivances  for  ensuring 
cross-fertilization  that  are  to  be  met  with  in  the  vegetable 
kingdom.  Many  of  these  have  been  described  and  illustrated  by 
Darwin  in  his  well-known  work  on  the  "  Fertilization  of  Orchids." 

In  the  common  early  orchis  of  Europe  (Orchis  masciila, 
Fig.  182)  there  are  three  sepals  and  three  petals,  and  both  sepals 
and  petals  are  brightly  coloured.  The  lower  petal  is  very  large  and 
in  front  forms  a  tongue-like  projection  termed  the  labellum  (r/), 
which  serves  as  a  landing  platform,  while  behind  it  is  produced 
backwards  into  a  hollow  spur  or  nectary  («p,  n)  in  which  honey 
is  secreted.  The  reproductive  organs  lie  just  above  and  partly 
in  front  of  the  entrance  to  the  nectary,  so  that  an  insect,  in 


360        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

poking  its  proboscis  down  to  get  at  the  honey,  cannot  fail  to  touch 
them.  Both  male  and  female  organs  are  very  much  modified  in 
structure,  and  united  together  to  form  the  column  (<7).  There 
are  three  stigmas.  Of  these,  however,  only  two  (#,  na)  are 
functional,  and  they  lie  at  the  back  of  the  entrance  to  the 
nectary.  The  third  is  specially  modified  to  form  a  remarkable 
organ  known  as  the  rostellum  (B,  C,  r),  which  projects  above  the 


FIG.  182. — Adaptation  in  the  Flower  of  Orchis  mascula  in  relation  to  Insect- 
Fertilization.     (From  Weismann's  "  Evolution  Theory.") 

A,  side  view  of  flower ;  B,  front  view ;  (7,  vertical  section  through  the  column ;  D,  pol- 
linia  removed  on  the  point  of  a  pencil  and  still  standing  erect ;  E,  the  same  later  on 
bent  forwards. 

ei,  entrance  to  nectary;  n,  nectary;  na,  stigmatic  surface;  p,  pollinia;  r,  rostellum 
Sm,  honey  guide ;  sp,  spur;  at.  ovary  (twisted) ;   U,  lower  lip  of  flower  (labellum). 

other  two  in  front  of  the  mouth  of  the  nectary.  This  rostellum, 
or  beak,  consists  of  an  exceedingly  sticky  body  covered  over  by  a 
thin,  membranous  cap.  The  cap  is  so  delicately  adjusted  that 
the  slightest  touch  is  sufficient  to  push  it  down  and  expose  the 
sticky  mass  beneath,  and  it  is  so  elastic  that  when  the  pressure 
is  removed  it  springs  back  into  its  original  position  and  again 
covers  up  the  sticky  substance. 

There  is  only  one  perfect  anther,  consisting  of  two  sacs  which 
stand  up  above  and  behind  the  rostellum.  Each  sac  contains  a 
single  coherent  mass  of  pollen  grains  instead  of  the  usual  loose, 


FERTILIZATION  OF  FLOWERS.  361 

powdery  pollen.  Each  mass  of  pollen  grains,  or  pollinium 
(7^,  C,  p),  is  a  pear-shaped  body  provided  with  a  short  stalk  or 
caudicle.  The  stalk  is  continued  downwards  into  the  rostellum 
as  shown  in  C,  where  it  ends  in  a  membranous  disc  attached 
to  the  sticky  substance.  When  the  pollen  is  ripe  the  pollinia  are 
exposed  by  rupture  of  the  sacs  in  which  they  lie,  so  that  in  the 
mature  flower  they  are  only  attached  by  means  of  their  stalks  or 
caudicles,  loosely  fixed  by  the  sticky  substance  of  the  rostellum. 

If  we  take  any  slender  object,  such  as  a  pencil  point,  and  poke 
it  gently  into  the  mouth  of  the  nectary,  we  shall  find  on  with- 
drawing it  again  that  it  will  bring  with  it  either  one  or  both  of 
the  pollinia,  attached  to  it  by  sticky  cement.  The  pencil  has,  of 
course,  touched  the  rostellum  and  pushed  down  the  little  cap 
which  covers  it.  The  sticky  substance  of  the  rostellum,  thus 
exposed,  has  cemented  on  to  the  pencil  the  disc  at  the  lower  end 
of  the  caudicle,  and  thus  the  pollinium  itself,  or  both  of  them,  is 
pulled  bodily  out  with  the  pencil.  The  cement  rapidly  hardens 
on  exposure  to  the  air. 

When  first  attached  to  the  pencil  the  pollinium  is  in  a  nearly 
upright  position  (D),  so  that  if  the  pencil  were  inserted  into 
another  flower  it  would  j- imply  go  back  into  its  old  place,  without 
touching  the  stigma  and  of  course,  therefore,  without  effecting 
fertilization.  After  being  exposed  to  the  air  for  a  short  time, 
however,  the  disc  by  which  the  pollinium  is  attached  to  the 
pencil  begins  to  contract,  and  in  such  a  manner  that  the  polli- 
nium is  pulled  down  until  it  comes  to  project  straight  forwards 
(E).  If  now  the  pencil  be  inserted  into  a  flower  it  will  be  found 
that  the  pollinium  exactly  strikes  against  one  of  the  stigmas  and 
dusts  it  with  pollen. 

In  the  economy  of  nature  the  proboscis  of  some  insect  takes  the 
place  of  our  experimental  pencil.  Bees  and  moths  have  fre- 
quently been  observed  with  the  pollinia  of  various  species  of 
Orchis  attached  to  them  ;  indeed  Darwin  figures  one  instance  in 
which  no  less  than  fourteen  pollinia  are  attached  to  the  proboscis 
of  a  moth.  Every  time  such  an  insect  visits  a  flower  in  search 
of  honey  it  will  effect  cross-fertilization  by  pushing  the  pollinia 
which  it  brings  with  it  against  the  stigmas,  and  may  perhaps 
carry  off  another  pollinium  into  the  bargain. 

In  this  case  one  can  hardly  fail  to  be  astonished  at  the  number 
and  complexity  of  the  adaptations  which  have  arisen  in  the 
flower  for  the  purpose  of  ensuring  cross-fertilization.  There  is 


362        OUTLINES    OF   EVOLUTIONARY   BIOLOGY 

the  conspicuous  colour  of  the  sepals  and  petals,  -which  serves  ns 
an  advertisement ;  the  formation  of  the  long  spur  or  nectary,  with 
its  secreted  honey ;  the  great  development  of  the  labellum  to 
serve  as  a  landing  place,  marked  with  the  "honey-guide  "  (Sm) 
to  point  out  the  way  to  the  visitor;  the  secretion  of  sticky 
cement  by  the  rostellum  ;  the  formation  of  an  elastic  cap  to  keep 
the  cement  from  being  dried  up  before  it  is  wanted;  the  agglu- 
tination of  the  pollen  into  solid  masses,  which  serve  to  fertilize 
a  large  number  of  flowers  in  succession,  losing  a  few  grains  at 
each  contact ;  the  remarkable  form  of  these  pollinia,  with  their 
adhesive  discs  and  their  peculiar  relation  to  the  sticky  substance 
on  the  rostellum  :  and,  lastly,  the  wonderful  mechanism  by  means 
of  which  the  polknia  become  bent  downwards  after  their  removal, 
so  as  exactly  to  adjust  them  for  c&ntact  with  the  stigma  of 
another  flower ! 

In  some  species  of  plants  certain  parts  of  the  flower  are  highly 
sensitive  and  respond  to  the"  slightest  touch  by  a  sudden  and 
vigorous  movement,  going  off,  so  to  speak,  like  a  spring  trap  or 
a  hair  trigger.  This  is  well  exemplified  in  a  New  Zealand  orchid, 
Ptcrostylis  trullifolia,  as  described  by  Mr.  Cheeseman.  In  this 
flower  the  lower  petal,  or  labellum,  is  highly  sensitive,  and  when 
an  insect  alights  upon  it  springs  up  and  imprisons  the  visitor 
in  a  cage.  The  parts  of  the  flower  are  so  arranged  that  the 
insect  can  only  escape  by  crawling  through  a  narrow  passage, 
and  in  such  a  manner  that  it  must  carry  away  the  pollinia  and 
also  leave  on  the  stigma  some  of  any  pollen  which  it  may  chance 
to  bring  with  it. 

A  still  more  remarkable  orchid,  Catasetum,  actually  throws  the 
pollinia  at  its  insect  visitors  as  soon  as  they  touch  the  flower, 
and  the  tiny  projectiles  attach  themselves  to  the  intruder's  head. 
Darwin  has  described  how,  on  one  occasion,  when  he  experi- 
mentally irritated  this  flower,  the  pollinium  was  thrown  for  a 
distance  of  nearly  three  feet,  when  it  stuck  on  to  a  window 
pane. 

The  presence  of  irritable  structures  which  aid  in  cross- 
*fertilization  by  insect  agency  is,  however,  by  no  means  confined 
to  orchids.  Ca-udollea  (Stylidwm)  gramimfoUa  is  a  common 
Australian  wild  flower  belonging  to  a  totally  different  order,  the 
Candolleacese.  It  is  found  growing  on  open  heaths  and  sends 
up  tall  stems  from  the  midst  of  a  tuft  of  long,  grass-like  leaves, 
each  stem  bearing  a  large  number  of  rather  small  pink  flowers 


FERTILIZATION  OF  FLOWERS.  368 

along  its  length.  There  are  five  petals,  but  only  four  are  fully 
developed,  and  these  spread  themselves  out  in  the  form  of  a 
cross  around  the  mouth  of  the  tube  in  which  the  honey  is 
secreted.  The  anthers  and  stigma  are  borne  on  the  end  of  a 
long  slender  column  which  springs  from  the  middle  of  the  flower. 
Close  to  its  base  the  column  is  bent  downwards  at  a  sharp  angle 
so  that  it  hangs  out  at  one  side  of  the  flower,  between  two  of 
the  fully  developed  petals  and  resting  upon  the  aborted  petal  as 
on  a  cushion.  Near  its  apex  the  column  is  again  bent  at  a  sharp 
angle,  this  time  upwards.  The  flowers  are  protandrous,  the 
anthers  becoming  mature  and  shedding  their  pollen  before  the 
stigma  is  fully  developed. 

An  insect,  visiting  the  flower  and  poking  its  proboscis  down 
the  tube  of  the  corolla  in  search  of  honey,  must  touch  the  column 
at  the  first  bend.  This  is  the  irritable  spot,  and  no  sooner  is  it 
touched  than  the  column  springs  over  to  the  other  side  of  the 
flower  and  brings  the  anthers  or  stigma,  as  the  case  may  be, 
down  on  the  back  of  its  visitor.  In  this  way  pollen  is  deposited 
upon  or  removed  from  the  insect's  back  and  cross-fertilization  is 
effected.  One  of  the  most  curious  things  about  this  case  is  that 
the  column  loses  its  irritability  at  nightfall  and  refuses  to  jump, 
however  much  you  may  ticlde  it. 

The  few  examples  of  entomophilous  flowers  which  we  have  now 
studied  must  suffice  to  give  some  idea  of  how  wonderfully  minute 
and  perfect  may  be  the  adaptation  of  plants  to  highly  specialized 
environmental  conditions.  The  dominating  factor  of  the  envi- 
ronment here  is,  of  course,  the  presence  of  insects,  which  can  be 
pressed  into  service  as  pollen  carriers,  and  it  is  obviously  in 
relation  to  the  requirements  and  tastes  of  these  insects  that  the 
various  adaptations  which  we  have  been  describing  have  arisen. 
There  can  be  little  doubt  that  the  insects  select  for  their  visits 
those  flowers  which  please  them  best  and  which  are  most  readily 
recognizable  as  the  bearers  of  the  coveted  honey. 

Although,  as  we  have  previously  pointed  out,  some  carrion- 
feeding  flies  prefer  scents  which  are  repulsive  to  ourselves,  yet 
the  vast  majority  of  insect-fertilized  flowers  have  scents  and 
colours  which  we  appreciate  perhaps  no  less  than  the  insects  for 
whose  gratification  they  primarily  exist.  As  we  shall  see  later  on, 
these  scents  and  colours,  though  now  in  many  cases  improved 
by  human  selection,  doubtless  arose  in  the  first  instance  in 
response  to  insect  selection.  If  we  pursue  this  line  of  thought  a 


364       OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

little  further,  there  seems  good  reason  to  suspect  that  man,  who 
appeared  upon  the  scene  at  a  very  much  later  date  and  whose 
ideas  of  what  is  beautiful  are  doubtless  largely  derived  from  the 
contemplation  of  flowers,  may  owe  much  of  his  aesthetic  develop- 
ment to  the  fact  that  he  has  been  educated  by  flower-loving 
insects. 


PART   V.— FACTORS   OF   ORGANIC   EVOLUTION 

CHAPTER  XXIV 

Views  of  Buffon,  Erasmus  Darwin  and  Lamarck. 

WE  shall  have  occasion  to  point  out  in  a  subsequent  chapter 
that  many  organisms  exhibit  characters  which  it  is  extremely 
difficult,  if  not  impossible,  to  bring  into  any  direct  relation- 
ship with  the  environment,  but  this  fact  does  not  invalidate 
the  generalization  that  the  most  striking  feature  of  all  living 
things  is  adaptation  to  the  conditions  under  which  they  have  to 
carry  on  their  existence.  In  seeking  for  an  explanation  of  the 
means  whereby  organic  evolution  has  been  effected,  this  fact 
must  constantly  be  borne  in  mind,  and,  as  we  have  already  said, 
no  theory  can  be  considered  adequate  which  does  not  take 
fully  into  account  the  phenomena  of  adaptation,  and  offer  some 
reasonable  explanation  of  the  wonderful  harmony  which  exists 
between  living  things  and  their  surroundings. 

We  have  said  before  that  the  theory  of  organic  evolution  is  no 
new  thing,  but  can  be  traced  back  even  to  the  ancient  Greek 
philosophers.  In  the  middle  ages  such  ideas  were  thrust  into 
the  background,  along  with  other  fruits  of  Greek_  and  Roman 
intellectual  activity,  and  to  a  large  extent  supplanted  by  the 
teachings  of  dogmatic  theology.  With  the  revival  of  scientific 
inquiry,  however,  the  theory  of  organic  evolution,  as  opposed  to 
the  doctrine  of  special  creation  and  the  immutability  of  species, 
again  began  to  occupy  a  prominent  place  in  the  minds  of 
thinking  men,  and  a  brief  consideration  of  the  views  of  some  of 
the  chief  philosophical  biologists  of  the  last  two  centuries  will 
perhaps  form  the  most  fitting  introduction  to  this  part  of  our 
subject. 

The  celebrated  French  naturalist,  Buffon  (1707—1788),  who 
held  the  post  of  Superintendent  of  the  Jardin  des  Plantes  and, 
in  conjunction  with  his  colleagues,  published  a  large  number  of 
volumes  of  Natural  History,  was  one  of  those  who  had  at  any 


366        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

rate  strong  leanings  towards  the  theory  of  organic  evolution, 
although,  unfortunately,  he  seems  to  have  found  it  very  difficult 
to  give  a  frank  expression  to  his  views.  The  following  trans- 
lated extracts  will  serve  to  illustrate  his  position. 

Speaking  of  the  arbitrary  character  of  our  systems  of  classifi- 
cation he  observes  :  — 

"  But  Nature  proceeds  by  unrecognized  gradations  and  con- 
sequently she  cannot  lend  herself  completely  to  these  divisions, 
since  she  passes  from  one  species  to  another  species,  and  often 
from  one  genus  to  another  genus,  by  imperceptible  shades :  so 
that  we  find  a  great  number  of  intermediate  species  and  objects 
which  we  do  not  know  where  to  place,  and  which  necessarily 
upset  the  plan  of  the  general  system."  1 

Buffon  certainly  appears  in  this  passage  as  no  believer  in  the 
immutability  of  species,  and  amongst  the  causes  which  bring 
about  their  modification  he  attributes  great  importance  to  the' 
action  of  climate: — 

"If  we  again  consider  each  species  in  different  climates,  we 
shall  find  obvious  varieties  both  as  regards  size  and  form  ;  all 
are  influenced  more  or  less  strongly  by  the  climate.  These 
changes  only  take  place  slowly  and  imperceptibly  ;  the  great 
workman  of  Nature  is  Time :  he  walks  always  with  even  strides, 
uniform  and  regular,  he  does  nothing  by  leaps  ;  but  by  degrees, 
by  gradations,  by  succession,  he  does  everything ;  and  these 
changes,  at  first  imperceptible,  little  by  little  become  evident, 
and  express  themselves  at  length  in  results  about  which  we 
cannot  be  mistaken."  2 

In  dealing  with  the  animals  of  the  old  and  the  new  worlds,  and 
after  speaking  of  the  extinction  of  the  mammoth,  he  says  :— 

"  This  species  was  certainly  the  foremost,  the  largest  and  the 
strongest  of  all  the  quadrupeds  :  inasmuch  as  it  has  disappeared, 
how  many  other  smaller  ones,  weaker  and  less  remarkable,  have 
had  to  succumb  also,  without  having  left  us  either  witness  or 
evidence  of  their  past  existence  ?  How  many  other  species,  having 
become  modified  in  their  nature,  that  is  to  say,  perfected  or 
degraded  by  the  great  vicissitudes  of  land  and  sea,  by  the  neglect 
or  the  culture  of  Nature,  by  the  long  influence  of  a  climate 
become  contrary  or  favourable,  are  no  longer  the  same  that  they 
formerly  were  ?  And,  moreover,  the  quadrupeds  are,  next  to 
man,  the  beings  whose  nature  is  the  most  fixed  and  whose  form 
is  the  most  constant :  that  of  the  birds  and  of  the  fishes  varies 

1  Buffon,  "  Histoire  Naturelle,"  Tom.  I,  p.  13. 
*  Op.  eit.,  Tom.  VI,  pp.  50,  60. 


more  ;  that  of  the  insects  still  more,  and  if  we  descend  to  the 
plants,  which  we  must  not  exclude  from  animated  nature,  we 
shall  be  surprised  at  the  promptitude  with  which  the  species 
vary,  and  at  the  facility  with  which  they  change  their  nature 
while  taking  on  new  forms. 

"  It  would  not  be  impossible  then,  that,  even  without  reversing 
the  order  of  Nature,  all  these  animals  of  the  new  world  may  have 
been  originally  the  same  as  those  of  the  old,  from  which  they 
may  have  been  formerly  derived;  one  might  say  that  having 
been  separated  subsequently  by  immense  seas  or  impassable 
lands,  they  would,  in  the  course  of  time,  have  received  all  the 
impressions,  suffered  all  the  effects,  of  a  climate  itself  altered  in 
character  by  the  same  causes  which  brought  about  the  separa- 
tion ;  that  in  consequence  they  would  in  time  have  become 
dwarfed,  changed  their  nature,  &c.  But  that  should  not  prevent 
us  from  regarding  them  to-day  as  animals  of  different  species  c* 
whatever  may  be  the  cause  of  this  difference,  whether  it  has  been 
produced  by  time,  climate  and  country,  or  whether  it  be  of  the 
same  date  as  the  creation,  it  is  none  the  less  real.  Nature,  I 
admit,  is  in  a  continual  state  of  flux ;  but  it  is  enough  for  man 
to  seize  her  as  she  is  in  his  own  time,  and  to  glance  backwards 
and  forwards  in  endeavouring  to  gain  some  glimpse  of  what  she 
may  have  been  in  former  times  and  of  what  she  may  become  in 
the  future."  l 

This  passage  also  seems  to  be  a  sufficiently  clear  declaration 
in  favour  of  the  theory  of  organic  evolution  and  the  action  of , 
the  environment  in  modifying  species.  Buffon,  however,  by  no 
means  confined  himself  to  the  consideration  of  climate  as  a 
factor  in  the  production  of  such  modifications.  The  following 
quotation  shows  that  he  also  realized  the  importance  of  the 
principles  of  use  and  disuse  and  the  inheritance  of  acquired 
characters,  which  were  destined  to  take  such  a  prominent  place 
in  the  subsequent  writings  of  Lamarck  :— 

"  The  llama,  which,  like  the  camel,  passes  its  life  in  bearing 
burdens  and  only  rests  by  lying  down  upon  its  breast,  has  similar 
callosities  which  are  perpetuated  in  the  same  way  by  generation. 
The  baboons  and  the  apes,  whose  most  usual  attitude,  whether 
awake  or  asleep,  is  sitting,  have  also  callosities  beneath  the  region 
of  the  buttocks,  and  this  callous  skin  has  even  become  adherent 
to  the  bones  against  which  it  is  continually  pressed  by  the  weight 
of  the  body ;  but  these  callosities  of  the  baboons  and  apes  are  dry 
and  healthy,  because  they  do  not  arise  from  the  constraint  of 
trammels  or  of  the  weight  of  a  foreign  burden,  and  because  they 

1  Op.  cit.,  Tom.  IX,  pp.  126,  127. 


368        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

are  merely  the  effects  of  the  natural  habits  of  the  animal,  which 
remains  seated  more  willingly  and  for  a  longer  time  than  in  any 
other  posture  :  it  is  the  same  with  these  callosities  of  the  apes  as 
with  the  double  sole  of  skin  which  we  carry  beneath  our  feet ; 
this  sole  is  a  natural  callosity  which  our  constant  habit  of  walking 
or  resting  upright  renders  more  or  less  thick,  or  more  or  less 
hard,  according  to  the  amount  of  friction  to  which  the  soles  of 
our  feet  are  exposed."  ] 

It  has  been  maintained  that  Buffon  not  only  anticipated 
Lamarck's  views  as  to  the  influence  of  the  environment  and  the 
principle  of  use  and  disuse,  but  also  those  of  Malthus  and 
Charles  Darwin  with  regard  to  the  importance  of  the  struggle  for 
existence  and  the  process  of  natural  selection.  That  some  such 
ideas  were  present  in  his  mind  seems  sufficiently  clear  from  the 
following  passages.  After  speaking  of  the  invasions  of  the  Huns 
and  Goths  and  other  peoples  he  continues  : — 

"  These  great  events,  these  conspicuous  epochs  in  the  history  of 
the  human  race,  are,  however,  only  trifling  vicissitudes  in  the 
ordinary  course  of  living  nature  ;  it  is  in  general  always  constant, 
always  the  same ;  its  movement,  always  regular,  turns  on  two 
fixed  pivots,  the  one  the  unlimited  fecundity  given  to  all  species, 
the  other  the  innumerable  obstacles  which  reduce  the  product  of 
this  fecundity  to  a  fixed  quantity  and  at  all  times  leave  only 
approximately  the  same  number  of  individuals  in  each  species."  2 

"  The  causes  of  destruction,  of  annihilation  and  sterility  follow 
immediately  upon  those  of  excessive  multiplication ;  and,  inde- 
pendently of  contagion,  the  necessary  consequence  of  too  great 
an  accumulation  of  any  living  matter  in  one  place,  there  are  in 
each  species  special  causes  of  death  and  destruction,  which  we 
shall  indicate  in  the  sequel  and  which  alone  suffice  to  compensate 
for  the  excess  of  former  generations. "  3 

"  The  least  perfect  species,  the  most  delicate,  the  most  heavily 
burdened,  the  least  active,  the  least  well  armed,  &c.,  have  already 
disappeared  or  will  disappear."  4 

Even  if,  on  the  strength  of  this  last  passage,  however,  we  can 
claim  that  Buffon  had  conceived  the  idea  of  the  survival  of  the 

1  Op.  cit.,  Tom.  XIV,  pp.  325,  326. 

2  Op.  cit.,  Tom.  VI,  p.  248. 
8  Ibid.,  p.  251. 

4  I  translate  this  passage,  which  I  have  not  found  in  the  original,  from  a  quotation 
given  by  Osborn  in  his  interesting  work  "  From  the  Greeks  to  Darwin  "  (Columbia 
University  Biological  Series).  The  student  should  refer  to  this  work  for  the  history 
of  the  theory  of  evolution. 


VIEWS   OF   BUFFON  369 

fittest,  the  mere  question  of  priority  is  a  matter  of  small  moment. 
As  a  matter  of  fact  it  is  now  well  known  that  this  idea  is  very 
much  older  than  Buffon,  and  can  be  traced    back,  as  Osborn 
remarks,  "  to  Ernpedocles,  six  centuries  before  Christ."  l 
Osborn  also  points  out  that  :  — 

"  Buffon's  ideas  regarding  the  physical  basis  of  heredity  are 
very  similar  to  those  of  Democritus,  and  certainly  contain  the 
basis  of  the  conception  of  the  Pangenesis  theory  of  Darwin,  for 
he  supposes  that  the  elements  oFtne  germ-cells  were  gatnered 
from  all  parts  of  the  body."  2 

So  far  we  have  presented  the  views  of  Buffon  as  those 
of  a  thoroughgoing  evolutionist.  He  had,  however,  apparently 
another  side  to  his  mind,  which  is  extremely  difficult  to  under- 
stand in  the  author  of  the  foregoing  passages*  In  the  fourth 
volume  of  his  Natural  History,  after  discussing  the  possible 
modification  of  species,  he  continues  :  — 

"  But  no,  it  is  certain,  by  revelation,  that  all  animals  have 
participated  equally  in  the  grace  of  creation,  that  the  two  first 
of  each  species  and  of  all  the  species  came  forth  complete  from 
the  hands  of  the  Creator  ;  and  we  must  believe  that  they  were 
then  much  the  same  as  they  are  now  represented  to  us  by  their 
descendants."3 

The  doctrine  of  special  creation  could  hardly  be  more  clearly 


Of  course  it  is  possible,  as  Samuel  Butler  suggests  in  his 
interesting  discussion  of  Buffon  in  "Evolution  Old  and  New," 
that  such  passages  as  this  may  be  ironical,  but  it  seems  more 
likely  that  Buffon  vacillated  between  what  was  then  regarded  as 
religious  orthodoxy  and  the  more  rational  views  which  he  knew 
so  well  how  to  express.  Indeed,  Butler  himself  remarks,  apropos 
of  another  passage  :  — 

"  This  is  Buffon's  way.  Whenever  he  has  shown  us  clearly 
what  we  ought  to  think,  he  stops  short  suddenly  on  religious 
grounds."  4 

1  Op.  cit.,  p.  117. 

2  Ibid.,  p.  135. 

8  "  Histoire  Naturelle,"Tom.  IV,  p.  383. 

4  "  Evolution  Old  and  New  "  (New  Issue,  London,  A.  &  C.  Fifield),  p.  115.  Thia 
work  contains  numerous  passages  translated  from  Buffon  and  Lamarck,  and  I 
have  found  it  of  great  use  as  a  guide  to  the  more  salient  passages  in  the  voluminous 
writings  of  the  former.  I  have,  however,  in  all  cases  made  fresh  translations  from 
the  French. 

B.  B  B 


870       OUTLINES  OF  E  VOLUTION  ABY  BIOLOGY 

Buffon's  inability  to  reconcile  the  logical  consequences  of  his 
own  inductions  with  his  religious  convictions  is  perhaps  nowhere 
better  illustrated  than  in  the  attitude  which  he  adopted  with 
regard  to  the  position  of  man  in  the  animal  kingdom.  Such  an 
experienced  observer  as  he  was  could  not  fail  to  realize  the  close 
agreement  in  structure  between  man  and  the  higher  apes  :  - 

"  We  shall  see,  in  the  history  of  the  orang  utan  that,  were  we  to 
pay  attention  only  to  the  form,  we  might  equally  well  look  upon 
this  animal  either  as  the  highest  of  the  apes  or  as  the  lowest 
of  mankind ;  because,  with  the  exception  of  the  soul,  he  lacks 
nothing  of  all  that  we  possess,  and  because  he  differs  less  from 
man  in  bodily  structure  than  he  differs  from  other  animals  to 
which  the  name  of  ape  has  been  given." l 

"  I  admit  that,  if  one  should  judge  only  by  form,  the  ape  species 
might  be  taken  for  a  variety  of  the  human  species  :  the  Creator 
did  not  think  fit  to  make  for  the  human  body  a  model  absolutely 
different  from  that  of  the  animal ;  He  has  included  his  form,  like 
that  of  all  the  animals,  in  one  general  plan  ;  but  at  the  same 
time  that  He  bestowed  upon  him  this  ape-like  material  form.  He 
penetrated  this  animal  body  with  His  divine  breath."2 

The  ape,  on  the  other  hand,  is  a  mere  animal,  and 

"  in  spite  of  his  resemblance  to  man,  far  from  being  the  second 
in  our  species,  he  is  not  the  first  in  the  order  of  animals,  since 
he  is  not  the  most  intelligent."  3 

There  can  be  no  doubt  that  the  views  of  Erasmus  Darwin 
(1731 — 1802)  were  largely  influenced  by  the  writings  of  Buffon,  to 
which  he  repeatedly  refers.  Darwin's  "  Zoonomia  "  was  published 
in  1794,4  but,  perhaps  because  it  was  mainly  a  medical  work,  it 
received  but  little  attention  from  professional  naturalists.  The 
section  dealing  with  "  Generation  "  contains  his  views  on  evolu- 
tion. A  few  quotations  will  suffice  to  illustrate  these  :— 

"  Owing  to  the  imperfection  of  language  the  offspring  is  termed 
a  new  animal,  but  is  in  truth  a  branch  or  elongation  of  the 
parent ;  since  a  part  of  the  em  bry on-animal  is,  or  was,  a  part  of 
the  parent ;  and  therefore  in  strict  language  it  cannot  be  said  to 

1  "  Histoire  Naturelle,"  Tom.  XIV,  p.  30.     The  name  "orang  utan  "  was  applied 
by  Buffon  to  the  chimpanzee. 

2  Ibid.,  p.  32. 
8  Ibid.,  p.  37. 

4  My  quotations  are  taken  from  an  edition  published  by  B.  Dugdale  at  Dublin 
in  1800. 


VIEWS   OF  ERASMUS   DARWIN  '  371 

be  entirely  new  at  the  time  of  its  production  ;  and  therefore  it 
inay  retain  some  of  the  habits  of  the  parent-system." 

Here  we  have  clearly  expressed  the  idea  of  continuity  which 
plays  such  an  important  part  in  modern  theories  of  heredity. 
Erasmus  Darwin  was,  however,  a  "  Spermatist,"  that  is  to  say 
he  believed 

"  that  the  embryon  is  produced  solely  by  the  male,  and  that  the 
female  supplies  it  with  a  proper  nidus,  with  sustenance,  and 
with  oxygenation;  and  that  the  idea  of  the  semen  of  the  male 
constituting  only  a  stimulus  to  the  egg  of  the  female,  exciting  it 
into  life,  (as  held  by  some  philosophers)  has  no  support  from 
experiment  or  analogy." 

It  must  be  remembered  that  he  wrote  in  the  days  before  the 
cell  theory  had  shed  its  illuminating  rays  over  the  science  of 
embryology  : — 

"I  conceive,"  says  he,  "the  primordiuna,  or  rudiment  of  the 
embryon,  as  secreted  from  the  blood  of  the  parent,  to  consist  of  a 
simple  filament  as  a  muscular  fibre," 

but  this  filament  was  not  necessarily  thread-like  in  form,  for 
he  adds  :— 

"  I  suppose  this  living  filament,  of  whatever  form  it  may  be, 
whether  sphere,  cube,  or  cylinder,  to  be  endued  with  the  capability 
of  being  excited  into  action  by  certain  kinds  of  stimulus." 

It  thus  absorbs  nutriment  and  becomes  organized  by  "accre- 
tion of  parts,"  and  this  leads  to  the  development  of  new  kinds 
of  "  irritability."  According  to  this  view  the  appearance  of  a 
new  organ  precedes  its  use  :— 

"  the  lungs  must  be  previously  formed  before  their  exertions  to 
obtain  fresh  air  can  exist." 

"From  hence  I  conclude,  that  with  the  acquisition  of  new 
parts,  new  sensations,  and  new  desires,  as  well  as  new  powers,  «,re 
produced  ;  and  this  by  accretion  to  the  old  ones,  and  not  by 
distention  of  them." 

But  the  exercise  of  these  new  powers  in  turn  gives  rise  to  the 
development  of  more  new  parts,  which 

"  are  formed  by  the  irritations  and  senmtions,  and  consequent 
exertions  of  the  parts  previously  existing^and  to  which  the  new 
parts  are  to  be  attached." 

"  From  this  account  of  reproduction  it  appears,  that  all  animals 
have  a  similar  origin,  viz.  from  a  single  living  filament ;  and 

B    B    2 


/ 


872        OUTLINES   OF  EVOLUTIONAKY  BIOLOGY 

that  the  difference  of  their  forms  and  qualities  has  arisen  only 
from  the  different  irritabilities  and  sensibilities,  or  voluntaries, 
or  associabilities,  of  this  original  living  filament ;  and  perhaps  in 
some  degree  from  the  different  forms  of  the  particles  of  the  fluids, 
by  which  it  has  been  at  first  stimulated  into  activity." 


"  From  their  first  rudiment,  or  primordium,  to  the  termina- 
tion of  their  lives,  all  animals  undergo  perpetual  transformations; 
which  are  in  part  produced  by  their  own  exertions  inconsequence 
of  their  desires  and  aversions,  of  their  pleasures  and  their  pains, 
or  of  irritations,  or  of  associations  ;  and  many  of  these  acquired 
forms  or  propensities  are  transmitted  to  th9ir  posterity." 

Dr.  Darwin  thus  passes  from  the  discussion  of  what  is  now 
termed  the  qrUogenv  or  development  of  the  individual  to  that  of 
the  phylogeny  or  development  of  the  race.  From  consideration 
of  the  former  he  endeavoured  to  gain  some  insight  into  the  latter, 
and  it  may  be  fairly  claimed  that  he  thus  anticipated  what  is 
known  in  modern  biology  as  the  Eecapitulation  Hypothesis  :  — 

"From  thus  meditating  on  the  great  similarity  of  the  struc- 
ture of  the  warm-blooded  animals,  and  at  the  same  time  of  the 
great  changes  they  undergo  both  before  and  after  their  nativity  ; 
and  by  considering  in  how  minute  a  portion  of  time  many  of 
he  changes  of  animals  above  described  have  been  produced  ; 

ould  it  be  too  bold  to  imagine,  that  in  the  great  length  of  time, 
since  the  earth  began  to  exist,  perhaps  millions  of  ages  before 
the  commencement  of  the  history  of  mankind,  would  it  be  too 
bold  to  imagine,  that  all  warm-blooded  animals  have  arisen  from 
one  living  filament,  which  the  great  First  Cause  endued  with  ani- 
mality,  with  the  power  of  acquiring  new  parts,  attended  with 
new  propensities,  directed  by  irritations,  sensations,  volitions 
and  associations  ;  and  thus  possessing  the  faculty  of  continuing 
to  improve  by  its  own  inherent  activity,  and  of  delivering  down 
those  improvements  by  generation  to  its  posterity,  world  without 
end  !  " 

Erasmus  Darwin  was  perfectly  familiar  with  the  idea  ©f 
adaptation,  as  manifested,  for  example,  in  the  colours  ®f 
animals  :— 

"  The  colours  of  many  animals  seem  adapted  to  their  purposes 
of  concealing  themselves  either  to  avoid  danger,  or  to  spring  upfn 
their  prey.  Thus  the  snake  and  wild  cat,  and  leopard,  are  s* 
coloured  as  to  resemble  dark  leaves  and  their  lighter  interstices*  ; 
birds  resemble  the  colour  of  the  brown  ground,  or  the  green 


VIEWS   OF   ERASMUS   DARWIN  373 

hedges,   which   they  frequent;   and  moths  and  butterflies  are 
coloured  like  the  flowers  which  they  rob  of  their  honey." 

"  A  proboscis  of  admirable  structure  has  been  acquired  by  the 
bee,  the  moth,  and  the  humming  bird,  for  the  purpose  of 
plundering  the  nectaries  of  flowers.  All  which  seem  to  have 
been  formed  by  the  original  living  filament,  excited  into  action 
by  the  necessities  of  the  creatures,  which  possess  them,  and  on 
which  their  existence  depends." 

In  the  following  sentences  we  have  at  any  rate  a  very  close 
approach  to  the  idea  of  natural  selection  which  forms  the  key- 
note of  the  evolutionary  theory  as  advocated  by  the  illustrious 
grandson  of  the  writer  :— 

"  A  great  want  of  one  part  of  the  animal  world  has  consisted 
in  the  desire  of  the  exclusive  possession  of  the  females  ;  and  these 
have  acquired  weapons  to  combat  each  other  for  this  purpose.  .  .  . 
So  the  horns  of  the  stag  are  sharp  to  offend  his  adversary,  but 
are  branched  for  the  purpose  of  parrying  or  receiving  the  thrusts 
of  horns  similar  to  his  own?  and -have  therefore  been  formed  for 
the  purpose  of  combating  other  stags  for  the  exclusive  posses- 
sion of  the  females  ;  who'  are  x>bserved,  like  the  ladies  in  the 
times  of  chivalry,  to  attend  the^&r"  t>f  the  victor." 

"  The  final  cause1  of  this  contest  amongst  the  males  seems  to  be, 
that  the  strongest  and  most  active  animal  should  propagate  the 
species,  which  should  thence  become  improved." 

Here  the  principle  of  selection  seems  to  be  clearly  enough 
recognized,  at  any  rate  that  form  thereof  which  Charles  Darwin 
afterwards  distinguished  as  sexual  selection. 


The  great  French  philosophical  biologist,2  Jean  Bap^iste 
Pierce  Antoine  de  Mon^et,  Chevalier  d$  LamarcK^was  born  in 
1744  and  died  in  1829.  His  most  celebrated  work,  the 
"  Philosophic  Zoologique,"  which  contains  the  fullest  expression 
of  his  mature  views  on  the  theory  of  organic  evolution,  was 
published  in  1809,  but  these  views  appear  to  have  been  first 
announced  to  the  world  in  the  opening  lecture  of  the  Course  of 
Zoology  given  at  the  Natural  History  Museum  in  Paris  in  1800, 

1  Cf.  Aristotle: — "The  final  cause,  that  for  the  sake  of  which  a  thing  exists*' 
(De  G-eneratione  Animalium,  Book  I,  71.VS    Clarendon  Press  Trans.). 
*  Lamarck  first  proposed  the  terra  Biolojry  for  the  Science  of  Living  Thi 


374        OUTLINES   OF   EVOLUTIONAKY   BIOLOGY 

and  published  in  the  following  year  in  the  form  of  a  preface  to 
the  "  Systeme  des  Animaux  sans  Vertebres."1 

It  seems  impossible  to  doubt  that  Lamarck,  like  Erasmus 
Darwin,  was  largely  influenced  in  his  views  by  the  writings  of 
his  great  compatriot  Buffon,  with  whom  he  was  on  terms  of 
personal  friendship,  and  to  whom  he  refers  in  his  published 
work.  Whether  or  not  he  was  acquainted  with  the  works  of 
Erasmus  Darwin  will  probably  never  be  known,  but  it  is  evident 
that  both  drew  part  at  any  rate  of  their  inspiration  from  the 
same  source.  Owing  largely  to  his  official  position  as  Professor 
of  Invertebrate  Zoology  in  the  National  Museum,  Lamarck  was 
able  to  bring  forward  a  very  imposing  array  of  facts  in  support 
of  his  opinions,  and  though  these  opinions  were  but  the  natural 
development  of  those  enunciated  by  his  predecessors,  he  was  able 
to  place  the  theory  of  organic  evolution  on  a  much  firmer  and 
broader  basis  than  it  had  previously  enjoyed.  He  also  doubtless 
derived  great  advantage  from  the  fact  that  he  was  an  expsrienced 
botanist  as  well  as  a  zoologist,  having  published  many  important 
botanical  works  before  he  turned  his  attention  more  particularly 
to  the  zoological  aspect  of  biology. 

During  the  earlier  part  of  his  life  Lamarck  appears  to  have 
accepted  the  still  prevalent  doctrine  as  to  the  immutability  of 
species,  and  it  is  perhaps  significant  that  his  conversion  to 
evolutionary  views  seems  to  have  followed  very  rapidly  upon  the 
extension  of  his  investigations  from  the  vegetable  to  the  animal 
kingdom. 

In  the  "  Philosophic  Zoologique  "  he  maintains  that  the  first 
living  things  arose  by  a  process  of  spontaneous  generation,  which 
may  still  take  place,  and  that  from  the  starting  points  thus  pro- 
vided the  entire  animal  and  vegetable  kingdoms  as  they  now  exist 
have  arisen  as  the  result  of  orderly  and  progressive  evolution. 

He  devotes  a  large  amount  of  space  to  the  question  of  classifi- 
cation and  the  conception  of  species,  and  arrives  at  the  following 
conclusions,  which  are,  on  the  whole,  in  singularly  close  agreement 
with  those  of  modern  biologists  : — 

"  But  these  classifications,  of  which  several  have  been  so  happily 
imagined  by  naturalists,  and  the  divisions  and  sub-divisions 

1  Vide  Packard's  "Lamarck,  the  Founder  of  Evolution,  his  Life  and  Work" 
(Longmans,  Green  &  Co.,  1901).  This  most  interesting  work  contains  translations  of 
portions  of  Lamarck's  writings,  and  has  for  the  first  time  made  the  work  of  the  great 
French  philosopher  available  to  the  general  reader  in  England  and  America.  I  have, 
however,  thought  it  desirable  to  give  fresh  translations  from  the  original  French. 


VIEWS   OF   LAMARCK  375 

which  they  present,  are  entirely  artificial  contrivances.  Nothing 
of  all  that,  I  repeat,  occurs  in  nature,  in  spite  of  the  foundation 
which  appears  to  be  given  to  them  by  certain  portions  of  the 
natural  series  which  are  known  to  us,  and  which  have  the 
appearance  of  being  isolated.  We  may  be  certain  that  amongst 
her  productions,  nature  has  really  formed  neither  classes,  nor 
orders,  nor  families,  nor  genera,  nor  constant  species,  but  only 
individuals  which  succeed  one  another  and  which  resemble 
those  which  produced  them.  Now  these  individuals  belong  to 
infinitely  diversified  races,  which  shade  off  under  all  forms  and 
in  all  degrees  of  organization,  and  each  of  which  maintains 
itself  without  change  so  long  as  no  cause  of  change  acts 
upon  it."  x 


"  The  name  species  has  been  given  to  every  collection  of 
similar  individuals  which  have  been  produced  by  other  individuals 
like  themselves. 

"  This  definition  is  exact ;  for  every  individual  that  enjoys 
life  always  resembles  very  closely  that  or  those  from  which  it 
sprang.  But  to  this  definition  has  been  added  the  supposition 
that  the  individuals  which  make  up  a  species  never  vary  in 
their  specific  character,  and  that  consequently  the  species  has- 
an  absolute  constancy  in  nature. 

"It  is  exactly  this  supposition  that  I  propose  to  combat, 
because  the  clear  evidence  obtained  by  observation  shews  that 
it  is  unfounded. 

"  The  supposition,  almost  universally  admitted,  that  living 
bodies  form  species  constantly  distinguished  by  invariable 
characters,  and  that  the  existence  of  these  species  is  as  old  as 
that  of  nature  herself,  was  established  at  a  time  when  obser- 
vations were  insufficient,  and  when  the  natural  sciences  were 
still  almost  non-existent.  It  is  always  contradicted  in  the  eyes 
of  those  who  have  seen  much,  who  have  for  a  long  time  followed 
nature,  and  who  have  profitably  consulted  the  great  and  rich 
collections  of  our  Museum."2 

After  speaking  of  the  doctrine  of  special  creation  Laniard^ 
continues : — 

"  Without  doubt,  nothing  exists  except  by  the  will  of  the 
sublime  Author  of  all  things.  But  can  we  assign  to  Him  laws 
in  the  execution  of  His  will,  and  fix  the  method  which  He  has 
followed  in  this  respect  ?  Has  not  His  infinite  power  been  able 
to  create  an  order  of  tilings  which  should  give  existence 

1  "  Philosophic  Zoologique,"  Tom.  I,  pp.  21,  22. 
*  Oft.  cit.,  Tom.  I,  pp.  54,  55. 


876        OUTLINES   OF   E VOLUTION AEY   BIOLOGY 

successively  to  everything  that  we  see,  as  to  everything  which 
exists  and  which  is  unknown  to  us  ? 

"Assuredly,  whatever  may  have  been  His  will,  the  immensity 
of  His  power  is  always  the  same  ;  and  in  whatever  manner  this 
supreme  will  may  have  been  executed,  nothing  can  diminish 
its  grandeur."1 

Lamarck  seems  to  have  been  the  first  to  insist  upon  the 
branching  character  of  evolutionary  series ;  after  speaking 
of  such  series  he  goes  on  : — 

"  I  do  not  wish  to  say  thereby  that  existing  animals  form  a 
very  simple  series,  everywhere  equally  graduated  ;  but  I  say 
that  they  form  a  branching  series,  irregularly  graduated,  and 
which  has  no  discontinuity  in  its  parts,  or  which,  at  least,  has 
nob  always  had,  if  it  be  true  that,  in  consequence  of  some  species 
having  been  lost,  such  discontinuity  occurs  anywhere.  It 
results  from  j-.Tn'fljjmt  k\)e±£pecies  which  termini 
ofthe_general  Iferles  are  connec  ted,  ~M  leajOCon—  one  sider  with 
otnleFlieighbolmng  species  which  shade  into  them."2 

T7ike   Buffon,   he   lays   great   stress  upon   the  action  of  the 
]   environment  in  modifying  organisms  : — 

"  Many  facts  teach  us  that  in  proportion  as  the  individuals  of 
one  of  our  species  change  their  situation,  their  climate,  their 
manner  of  living  or  their  habits,  they  thereby  receive  influences 
which  little  by  little  change  the  consistency  and  the  proportions 
of  their  parts,  their  form,  their  faculties,  even  their  organization  ; 
so  that  everything  in  them  participates,  in  the  course  of  time,  in 
the  transformations  which  they  experience. 

"  In  the  same  climate,  very  different  situations  and  exposures 
at  first  cause  the  individuals  exposed  thereto  simply  to  vary ; 
but,  in  the  course  of  time,  the  continual  difference  in  situation  of 
the  individuals  of  which  I  am  speaking,  which  live  and  reproduce 
themselves  successively  in  the  same,  circumstances,  causes  in 
them  differences  which  become,  in  some  way,  essential  to  their 
existence ;  so  that,  after  many  generations  have  succeeded  one 
another,  these  individuals,  which  belonged  originally  to  another 
species,  find  themselves  in  the  end  transformed  into  a  new  species, 
distinct  from  the  other. 

"For  example,  if  the  seeds  of  a  grass,  or  of  any  other  plant 
natural  to  a  damp  meadow,  be'  transported  by  any  chance,  first 
to  the  slope  of  a  neighbouring  hill,  where  the  soil,  although  more 
elevated,  is  still  sufficiently  cool  to  permit  of  the  plant  main- 
taining itself,  and  if  afterwards,  after  having  lived  there  and 

1   Op.  cit.,  Tom    I,  pp.  5G,  57. 
?  IUA.,  p.  fill. 


VIEWS   OF   LAMARCK  377 

reproduced  itself  many  times,  it  reaches,  by  slow  degrees,  the 
dry  and  almost  arid  soil  of  a  mountainous  region ;  if  the  plant 
succeeds  in  living  there,  and  perpetuates  itself  for  a  succession 
of  generations,  it  will  then  become  so  changed  that  the  botanists 
who  come  across  it  will  make  of  it  a  distinct  species."  * 

As  Lamarck's  views  have  so  frequently  been  misrepresented  it 
is  incumbent  upon  us  to  make  ourselves  thoroughly  acquainted 
with  what  he  really  meant  by  the  action  of  the  environment,  and 
on  this  point,  fortunately,  he  is  very  precise  : — 

"  Here  it  becomes  necessary  for  me  to  explain  the  meaning 
which  I  attach  to  these  expressions  :  The  environment  (lea 
circonstances)  influences  the  form  and  organization  of  animals,  or 
in  other  words,  as  it  becomes  very  different  it  changes,  in  course 
of  time,  the  form  and  organization  themselves  by  proportional 
modifications. 

"  Certainly,  if  people  took  these  expressions  literally  they 
would  attribute  to  me  an  error ;  for,  whatever  the  environment 
may  be,  it  does  not  directly  effect  any  modification  whatever  in 
the  form  and  organization  of  animals. 

"  But  great  changes  in  the  environment  lead,  in  the  case  of 
animals,  to  great  changes  in  their  requirements  (besoins),  and 
such  changes  in  their  requirements  lead  necessarily  to  actions. 
Now,  if  the  new  requirements  become  constant  or  very  lasting, 
the  animals  then  adopt  new  liabits,  which  are  as  lasting  as  the 
requirements  which  have  called  them  forth.  .  .  . 

"Now,  if  a  novel  environment,  having  become  permanent  for 
a  race  of  animals,  has  given  to  these  animals  new  habit*,  or  in 
other  words  has  led  them  to  new  actions  which  have  become 
habitual,  there  will  have  resulted  therefrom  the  use  of  some 
particular  part  [of  the  body]  in  preference  to  some  other,  and  in 
certain  cases  the  complete  disuse  of  some  part  which  has  become 
useless. 

"  None  of  these  statements  should  be  considered  as  hypo- 
thetical or  as  the  expression  of  individual  opinion ;  they  are,  on 
the  contrary,  truths  which  need  only  attention  and  the  observa- 
tion of  facts  to  render  them  evident. 

"  We  shall  see  immediately,  by  the  citation  of  known  facts 
which  attest  it,  on  the  one  hand  that  new  requirements  having 
made  some  part  necessary,  have  really,  by  a  series  of  efforts, 
caused  this  part  to  arise,  and  that  afterwards  its  continued 
employment  has  little  by  little  strengthened  it,  developed  it,  and 
in  the  end  considerably  enlarged  it ;  on  the  other  hand,  we  shall 
see  that,  in  certain  cases,  the  uovel  environment  and  new 
requirements  having  rendered  some  part  quite  useless,  the 

>   f)p,  cit.,  Tom,  T,  pp.  62,  63, 


378        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

complete  want  of  employment  of  this  part  has  caused  it  to 
gradually  cease  from  developing  like  the  other  parts  of  the 
animal ;  that  it  has  become  reduced  and  attenuated  little  by 
little,  and  that  at  length,  when  this  want  of  employment  has 
been  complete  during  a  long  period,  the  part  in  question  has  in 
the  end  disappeared.  All  this  is  positive  ;  I  propose  to  give  the 
most  convincing  proofs  of  it. 

"In  plants,  where  there  are  no  actions,  and  consequently  no 
habits  properly  so-called,  great  changes  in  the  environment  have 
none  the  less  led  to  great  differences  in  the  development  of  their 
parts  ;  in  such  a  way  that  these  differences  have  caused  certain 
of  them  to  appear  and  develop,  while  they  have  caused  many 
others  to  dwindle  away  and  disappear.  B;it  here  everything  is 
effected  by  changes  which  take  place  in  the  nutrition  of  the 
plant,  in  its  absorptions  and  transpirations,  in  the  amount  of 
heat,  light,  air  and  moisture,  which  it  then  habitually  receives ; 
finally,  in  the  superiority  which  certain  of  the  various  vital 
movements  may  acquire  over  others."1 

Like   his  predecessors,    and    like    those   who   followed   him, 
marck   adduces   in   support  of    these  views   the   remarkable 
edifications  which  have   taken  place  in  animals   and  plants 
under  the  influence  of  domestication  : — 

"  That  which  nature  does  with  the  aid  of  much  time,  we  do 
every  day  by  suddenly  changing,  in  relation  to  some  living  plant, 
the  conditions  under  which  it  and  all  the  individuals  of  its  species, 
have  existed. 

"  All  botanists  know  that  the  plants  which  they  transport  from 
their  native  place  in  order  to  cultivate  them  in  gardens,  undergo, 
little  by  little,  changes  which,  in  the  end,  make  them  unrecogniz- 
able. .  .  . 

"Is  not  the  cultivated  wheat  (Triticum  sativum)  a  plant 
brought  by  man  to  the  condition  in  which  we  now  actually  see 
it?  Who  will  tell  me  in  what  country  such  a  plant  occurs 
naturally,  that  is  to  say  except  as  the  result  of  its  cultivation  in 
some  neighbouring  place  ? 

"  Where  do  we  find,  in  a  state  of  nature,  our  cabbages,  our 
lettuces,  &c.,  as  we  now  possess  them  in  our  kitchen  gardens  ? 
Is  it  not  the  same  with  respect  to  the  many  animals  which 
domestication  has  changed  or  considerably  modified?"  ' 

We  may  next  consider  two  examples  of  the  kind  of  evidence 
which  Lamarck  brings  forward  in  proof  of  the  effects  of  the 

1  Op.  oit.,  Tom.  1   pp.  22 1  —223. 

2  Ibid.,  pp.  226-227.  v 


VIEWS   OP   LAMAECK  370 

environment  and  of  the  use  and  disuse  of  organs  upon  plants 
and  animals  living  in  a  state  of  nature  : — 

"  The  following  fact  proves,  with  regard  to  plants,  how  much 
the  alteration  of  any  important  factor  in  the  environment 
modifies  the  parts  of  these  living  bodies. 

"  So  long  as  Ranunculus  aquatilis  is  sunk  beneath  the  surface 
of  the  W7ater,  its  leaves  are  all  finely  divided  and  have  their 
divisions  hair-like  ;  but  when  the  stems  of  this  plant  reach  the 
surface  of  the  water,  the  leaves  which  develop  in  the  air 
become  broadened,  rounded  and  simply  lobed.  If  some  runners 
of  the  same  plant  succeed  in  pushing  their  way  into  a  soil  which 
is  merely  damp,  without  being  inundated,  their  stems  are  then 
short,  and  none  of  their  leaves  are  divided  into  hair-like 
segments ;  thus  arises  Ranunculus  hederaceus,  which  botanists 
regard  as  a  species  when  they  come  across  it." l 

"  With  regard  to  habits,  it  is  curious  to  observe  the  result 
thereof  in  the  remarkable  form  and  in  the  stature  of  the  giraffe 
(Camelo-pardcdis) :  we  know  that  this  animal,  the  tallest  of  the 
mammals,  inhabits  the  interior  of  Africa,  and  that  it  lives  in 
places  where  the  earth,  almost  always  arid  and  without  herbage, 
obliges  it  to  browse  on  the  foliage  of  trees  and  to  make  con- 
tinual efforts  to  reach  it.  As  a  result  of  this  habit,  maintained 
for  a  long  time  in  all  the  individuals  of  its  race,  the  fore  limbs 
have  become  much  longer  than  the  hind  ones,  and  the  neck  has 
become  so  much  elongated  that  the  giraffe,  without  standing 
up  on  its  hind  legs,  raises  its  head  and  reaches  a  height  of 
six  metres  (nearly  twenty  feet)."  2 

The  following  passage  must  suffice  to  give  some  idea  of 
Lamarck's  views  on  the  inheritance  of  "  acquired  "  characters, 
which  his  theory  necessarily  implies,  though  not  in  the 
exaggerated  sense  of  some  modern  writers  :— 

"  These  familiar  facts  are  surely  well  suited  to  prove  what  is 
the  result  of  the  habitual  use  by  animals  of  some  particular 
organ  or  part ;  and  if,  when  we  observe,  in  an  animal,  an  organ 
specially  developed,  strong  and  powerful,  anyone  pretends  that 
its  habitual  exercise  has  caused  it  to  gain  nothing,  that  its 
continued  disuse  would  cause  it  to  lose  nothing,  and  that,  in 
short,  this  organ  has  always  been  as  it  is  since  the  creation  of 
the  species  to  which  the  animal  belongs,  I  would  ask  why  our 
domestic  ducks  can  no  longer  fly  like  wild  ducks  ;  in  a  word,  I 
would  cite  a  multitude  of  examples  relative  to  ourselves,  which 
bear  witness  to  the  differences  which  result  in  our  own  bodies 

1  Op.  cit.,  Tom.  I,  p.  230. 
a  Ibid.,  pp.  256—257. 


." 


380        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

from  the  exercise  or  the  want  of  exercise  of  any  of  our  organs, 
although  these  differences  are  not  maintained  in  the  individuals 
of  the  next  generation,  for  then  their  results  would  be  much 
more  considerable. 

"  I  shall  show  in  the  second  part  that  when  the  will  determines 
an  animal  to  some  action,  the  organs  which  have  to  execute  this 
action  are  forthwith  stimulated  by  the  affluence  of  subtle  fluids 
(the  nervous  fluid)  which  become  the  determining  cause  of  the 
movements  which  the  action  in  question  requires.  A  multitude 
of  observations  establishes  this  fact,  which  should  no  longer  be 
called  in  question. 

"tt  results  therefrom  that  numerous  repetitions  of  these  acts 
of  organization  strengthen,  extend,  develop  and  even  create 
the  organs  which  are  needed.  It  is  only  neeessary  to  observe 
ttentively  what  is  happening  everywhere  in  this  respect  to 
convince  oneself  of  the  actuality  of  this  cause  of  organic 
development  and  modification. 

fcbw,  every  change  acquired  in  an  organ  by  a  habit  of  use 
•ent   to   have   produced   it,  is   maintained   afterwards    by 
•ation,  if  it  is  common  to  the  individuals  which  have  united 
Re  act  of  fecundation  for  the  reproduction  of  their  species, 
ort,  this  modification  is  propagated,  and  thus  passes  to  all 
individuals  which  follow  and  which   are   subjected   to  the 
environment,  without  their  being  obliged  to  acquire  it  by 
means  which  really  created  it. 

~he  mingling  in  reproductive  unions,  however,  between 
iduals  which  have  different  qualities  or  forms,  is  necessarily 
to  the  constant  propagation  of  those  qualities  and  forms. 
This  is  the  reason  why  in  man,  who  is  subjected  to  so  many 
different  modifying  circumstances,  the  accidental  qualities  or 
defects  which  he  has  chanced  to  acquire  are  not  preserved  and 
propagated  by  generation.  If,  when  any  peculiarities  of  form  or 
any  defects  have  been  acquired,  two  individuals  in  this  condition  1 
should  always  unite,  they  would  reproduce  the  same  peculiarities,,! 
and  successive  generations  confining  themselves  to  similar^ 
unions,  a  special  and  distinct  race  would  then  be  formed.  But 
the  perpetual  .mingling  between  individuals  which  have;  not  the 
same  peculiarities  of  form  causes  all  peculiarities  acquired  as  the 
result  of  peculiar  circumstances  of  the  environment  to  disappear. 
Whence  one  may  be  certain  that  if  human  beings  were  not 
separated  by  the  distances  of  their  habitations,  the  mixed  breed - 
ntj  would  cause  the  general  characters  which  distinguish  the 
different  nations  to  disappear."  l 

In  the  light  of  modern  knowledge  these  views  on  the  subject 
of  heredity  are,  of  course,  crude  and  inaccurate  enough,  but  there 

1  Oj>.  cit.}  Tom.  I,  pp.  259—262. 


VIEWS   OF   LAMARCK  381 

is  nothing  absurd  in  them,  and  at  the  time  when  Lamarck  wrote 
it  would  scarcely  have  been  Mfssible  to  formulate  anything 
better. 

It  is,  however,  this  assumption  of  the  inheritance  of  somato- 
genic  characters  that  has  probably  done  more  than  anything  else 
to  prevent  many  modern  biologists  from  accepting  the  so-called 
Lamarckian  factors  of  evolution.  Those  who  hold  with  Weis- 
mann  that  there  is  no  possible  mechanism  by  which  a  somato- 
genic  character  can  be  coyerted  into  a  blastogenic  one  are 
forced  to  reject  Larnarck'dBteaching,  but  the  Weismannian 
assumption  that  there  is  no  such  possibility  of  inheritance  of 
sornatogenic  characters  rests  upon  no  better  foundation  than  the 
Lamarckian  assumption  thatjfchere  is.  We  have  dealt  with  this 
question  at  some  length  in  al  earlier  chapter  and  need  only  now 
remember  that,  if  there  areMnany  modern  biologists  who  reject 
the  Lamarckian  facto^b^Iise  they  cannot  reconcile  them  with 
Weismannism,  there  are  p^fcably  quite  as  many  others  who 
accept  them  because  they  l^peal  irresistibly  to  their  common 
sense,  and  because  they  refuse  to  believe\that  Weismann's 
difficulties  are  really  insurmountable. 

It  is,  of  course,  easy  to  ridicule  Lamarck's  views,  and  to  say, 
for  example,  that  he  maintained  that  an  animal  could  develop 
an  organ  by  simply  wishing  for  it,  and  ridicule 'of  this  kind  has 
undoubtedly  done  much  to  hinder  the  due  appreciation  of  his 
work.  Such  statements,  however,  are  only  made  by  those  who 
have  never  paid  adequate  attention  to  the  writings  of  the  great 
French  biologist. 

It  is  evident  from  the  passage  last  quoted  that  Lamarck  also 
did  not  fail  to  perceive  the  importance  of  isolation  as  a  factor  in 
organic  evolution,  necessary  to  prevent  the  swamping  effects  of 
intercrossing  upon  newly  arisen  species  or  varieties. 

In  another  place  he  discusses  the  influence  of  the  struggle  for 
existence  in  counteracting  the  effects  of  excessive  multiplication, 
and  in  so  doing  just  misses  the  idea  of  Natural  Selection: — 

"  Animals  eat  one  another,  except  those  which  live  only  upon 
plants ;  but  the  latter  are  liable  to  be  devoured  by  carnivores. 

"  We  know  that  it  is  the  stronger  and  the  better  armed  which 
eat  the  weaker,  and  that  the  large  species  devour  the  smaller 
ones.  Nevertheless  the  individuals  of  one  and  the  same  race 
rarely  eat  each  other ;  they  make  war  on  other  races."  l 

1  Oy.  cit.,  Tom.  I,  p.  99. 


382        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Lamarck  summed  up  his  views  as  to  the  factors  which  have 
co-operated  in  organic  evolution,  at  any  rate  so  far  as  the  animal 
kingdom  is  concerned,  in  a  later  work,  the  "  Histoire  Naturelle 
des  Animaux  sans  Vertebres,"  published  in  1815,  in  the  form  of 
four  "  Laws,"  which  may  be  taken  as  replacing  the  two  "  Laws  " 
of  the  "  Philosophie.Zoologique."  They  are  as  follows  : — 

"First  Law:  Life,  by  its  own  forces,  tends  continually  to 
increase  the  volume  of  every  body  which  possesses  it,  and  to 
extend  the  dimensions  of  its  parts,  up  to  a  limit  which  life 
itself  imposes."1 

"  Second  Law  :  The  production  of  a  new  organ  in  an  animal 
body  results  from  a  new  requirement  which  continues  to  make 
itself  felt,  and  from  a  new  movement  which  this  requirement 
begets  and  maintains."2 

"  Third  Law :  The  development  and  efficiency  of  organs  are 
constantly  in  proportion  to  the  use  of  these  organs."3 

"  Fourth  Law :  All  that  has  been  acquired,  traced  out  or 
altered  in  the  organization  of  individuals  during  the  course  of 
their  life,  is  preserved  by  generation,  and  transmitted  to  the 
new  individuals  which  originate  from  those  which  have 
experienced  these  modifications."4 

With  regard  to  the  position  of  man  in  the  animal  kingdom 
Lamarck,  unfortunately,  does  not  seem  to  have  been  able,  any 
more  than  Buffon,  to  divest  himself  of  the  fetters  of  religious 
orthodoxy.  After  pointing  out  at  length  the  numerous  ties  by 
which  man,  in  his  bodily  organization,  is  united  to  the  lower 
animals,  and  especially  his  close  relationship  to  the  apes,  he 
saves  himself,  so  to  speak,  in  the  following  paragraph  :— 

11  Such  would  be  the  reflections  which  one  might  make  if  man, 
considered  here  as  the  pre-eminent  race  in  question,  were  only 
distinguished  from  the  animals  by  the  characters  of  his 
organization,  and  if  his  origin  were  not  different  Trom  theirs."5 

1  "  Histoire  Naturelle  des  Animaux  sans  Vertebres,"  Tom.  I,  1815,  p.  182. 

2  Ibid.,  p.  185. 
8  Ibid.,  p.  189. 
*  Ibid.,  p.  199. 

'-  "  Philosophic  Zoologique,"  Tom.  I,  p.  357.    , 


V 


CHAPTER  XXV 

Bobert/vtihambers  and  the  "Vestiges  of   Creation" — Natural  Selection— 
The  Views  of  Charles  Darwin  and  Alfred  Eussel  Wallace. 

FOR  half  a  century  after  the  appearance  of  the  "  Philosophic 
Zoologique  "  the  theory  of  organic  evolution  made  but  little 
progress.  The  gap,  however,  was  to  some  extent  filled  by  the 
publication,  at  first  anonymously,  of  Robert  Chambers'  celebrated 
book,  "Vestiges  of  the  Natural  History  of  Creation."  This  work 
first  appeared  in  the  year  1844  and  rapidly  passed  through  a  large 
number  of  editions.  Though  the  views  of  its  author  can  hardly 
be  said  to  mark  any  advance,  but  on  the  whole  perhaps  rather  a 
retrogression,  the  work,  which  gave  rise  to  much  controversy, 
undoubtedly  played  a  very  important  part  in  preparing  the  way 
for  the  reception  of  Charles  Darwin's  "Origin  of  Species." 
Chambers  presented  the  evidence  of  organic  evolution  in  a  very 
convincing  manner,  laying  great  stress  upon  that  afforded  by 
the  geological  record  and  the  facts  of  comparative  anatomy  and 
embryology,  and  he  included  mankind  in  his  general  scheme  of 
evolution. 

His  views  as  to  the  modus  operandi  of  organic  evolution  are 
probably  expressed  as  clearly  as  such  views  could  be  in  the 
following  paragraphs : — 

"  The  proposition  determined  on  after  much  consideration  is^ 
that  the  several  series  of  animated  beings,  from  the  simplest  and 
oldest  up  to  the  highest  and  most  recent,  are,  under  the  providence 
of  God,  the  results,  first,  of  an  impulse  which  has  been  imparted 
to  the  forms  of  life,  advancing  them,  in  definite  times,  by  genera- 
tion, through  grades  of  organization  terminating  in  the  highest 
dicotyledons  and  vertebrata,  these  grades  being  few  in  number, 
and  generally  marked  by  intervals  of  organic  character  which 
we  find  to  be  a  practical  difficulty  in  ascertaining  affinities  ; 
second,  of  another  impulse  connected  with  the  vital  forces, 
tending,  in  the  course  of  generations,  to  modify  organic  structures 
in  accordance  with  external  circumstances,  as  food,  the  nature 
of  the  habitat  and  the  meteoric  agencies,  these  being  the 


384        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

"  adaptations  "  of  the  natural  theologian.  We  may  contemplate 
these  phenomena  as  ordained  to  take  place  in  every  situation, 
and  at  every  time,  where  and  when  the  requisite  materials  and 
conditions  are  presented — in  other  orbs  as  well  as  in  this — in 
any  geographical  area  of  this  globe  which  may  at  any  time 
arise — observing  only  the  variations  due  to  difference  of 
materials  and  of  conditions."1 

"  For  the  history,  then,  of  organic  nature,  I  embrace,  not  as  a 
proved  fact,  but  as  a  rational  interpretation  of  things  as  far  as 
science  has  revealed  them,  the  idea  of  Progressive  Development. 
We  contemplate  the  simplest  and  most  primitive  types  of  being, 
as  giving,  under  a  law  to  which  that  of  like-production  is  sub- 
ordinate, birth  to  a  type  superior  to  it  in  compositeness  of 
organization  and  endowment  of  faculties;  this  again  producing 
the  next  higher,  and  so  on  to  the  highest.  We  contemplate,  in 
short,  a  universal  gestation  of  nature,  analogous  to  that  of  the 
individual  being ;  and  attended  as  little  by  circumstances  oi 
a  startling  or  miraculous  kind,  as  the  silent  advance  of  an 
ordinary  mother  from  one  week  to  another  of  her  pregnancy. 
We  see  but  the  chronicle  of  one  or  two  greatjireas,  within  which 
the  development  has  reached  the  highest  forms.  In  some  others, 
as  Australia  and  the  islands  of  the  Pacific,  development  appears 
to  have  not  yet  passed  through  the  whole  of  its  stages,  because, 
owing  to  the  comparatively  late  uprise  of  the  land,  the  terrestrial 
portion  of  the  development  was  there  commenced  more  recently. 
It  would  commence  and  proceed  in  any  new  appropriate  area,  on 
this  or  any  other  sphere,  exactly  as  it  commenced  upon  our  area 
in  the  time  of  the  earliest  fossiliferous  rocks,  whichever  these 
are.  Nay,  it  perhaps  starts  every  hour  with  common  infusions, 
and  in  similar  humble  theatres,  and  might  there  proceed  through 
all  the  subsequent  stages,  granting  suitable  space  and  conditions. 
Thus  simple — after  ages  of  marvelling — appears  Organic 
Creation,  while  yet  the  whole  phenomena  are,  in  another  point 
of  view,  wonders  of  the  highest  kind,  being  the  undoubted  results 
of  ordinances  arguing  the  highest  attributes  of  foresight,  skill, 
and  goodness  on  the  part  of  their  Divine  Author." 2 

Progressive  evolution  is  here  clearly  attributed  to  some  inherent 
tendency  implanted  in  the  first  living  things,  and  apparently  the 
writer  imagines  that  the  same  or  closely  similar  results  may 
have  been  arrived  at  along  many  different  lines  of  evolution,  each 
commencing  at  a  distinct  starting  point  and  at  a  different  time  and 

1  "Vestiges  of  the  Natural  History  of  Creation,"  12th  ed.,  1884,  pp.  201,  202. 
*  Ibid.,  pp.230,  231. 


NATURAL   SELECTION  885 

place  from  all  the  others.  The  author  of  this  hypothesis  evidently 
considers  it  a  distinct  improvement  upon  the  views  of  Lamarck, 
which  he  very  briefly  discusses.  He  thinks  that  Lamarck  attributes 
too  much  importance  to  the  principle  of  use  and  disuse,  which  he 
regards  as  "  obviously  insufficient  to  account  for  the  great  grades 
of  organization,"  though  he  admits  that  external  conditions  may 
have  been  "a  means  of  producing  the  exterior  characters." 
There  can  be  little  doubt,  however,  that,  in  relying  upon  a  system 
of  more  or  less  definite  and  continuously  operating  natural 
causes  as  factors  of  organic  evolution,  Lamarck  took  up  a  far 
more  scientific  position  than  Robert  A6hambers. ! ! 


In  insisting  upon  the  great  importance  of  Natural  Selection  as 
a  factor  in  organic  evolution,  Charles  Darwin  and  Alfred  Russel 
Wallace  made  a  great  advance  upon  the  position  of  any  of  their 
predecessors. 

We  have  seen  in  the  last  chapter  that  this  principle  had  been 
hinted  at  by  more  than  one  writer  about  the  close  of  the  eighteenth 
and  the  commencement  of  the  nineteenth  centuries,  and  that  it 
can  even  be  traced  back  to  the  philosophy  of  ancient  Greece.  In 
the  historical  sketch  which  prefaces  the  later  editions  of  the 
"  Origin  of  Species,"  Charles  Darwin  himself  quotes  a  translation 
from  Aristotle  which  shows  sufficiently  clearly  that  the  idea  was 
familiar  to  the  great  Greek  biologist.  In  the  same  sketch  he 
also  quotes  a  passage  from  Dr.  W.  C.  Wells,  from  a  paper  read 
before  the  Royal  Society  in  1813,  in  which  the  same  principle  is 
recognized  in  the  most  explicit  and  unmistakable  manner. 

It  was  not,  however,  until  the  year  1858  that  the  part  played 
by  natural  selection  in  organic  evolution  began  to  be  gene- 
rally understood.  In  that  year  Sir  Charles  Lyell  and  Dr.  J.  D. 
Hooker  communicated  to  the  Linnean  Society  certain  papers,1 
written  by  Darwin  and  Wallace,  which  at  once  called  prominent 
attention  to  the  importance  of  this  factor  and  contained  the 
essential  parts  of  the  theory  of  natural  selection  as  subsequently 
developed  by  both  these  writers. 

Darwin's  paper  consisted  of  an  extract  from  his  as  yet 
unpublished  work,  together  with  an  abstract  of  a  letter  to 

1  These  papers  have  been  reprinted  by  the  Linnean  Society  in  the  volume  pub- 
lished in  connection  with  the  Darwin-Wallace  Celebration  held  on  July  1st,  1908, 
and  it  is  from  this  volume  that  the  quotations  which  follow  are  taken. 

B.  CO 


\ 


386        OUTLINES   OF  EVOLUTIONARY  BIOLOGY 

Professor  Asa  Gray.     A  few  quotations  will  suffice  to  indicate 
his  views : — 

"  De  Candolle,  in  an  eloquent  passage,  has  declared  that  all 
nature  is  at  war,  one  organism  with  another,  or  with  external 
nature.  ...  It  is  the  doctrine  of  Malthus  applied  in  most 
cases  with  tenfold  force.  .  .  .  Even  slow-breeding  mankind 
has  doubled  in  twenty-five  years ;  and  if  he  could  increase  his 
food  with  greater  ease,  he  would  double  in  less  time.  But  for 
animals  without  artificial  means,  the  amount  of  food  for  each 
species  must,  on  an  average,  be  constant,  whereas  the  increase  of 
all  organisms  tends  to  be  geometrical,  and  in  a  vast  majority  of 
cases  at  an  enormous  ratio.  Suppose  in  a  certain  spot  there 
are  "eight  pairs  of  birds,  and  that  only  four  pairs  of  them 
annually  (including  double  hatches)  rear  only  four  young,  and 
that  these  go  on  rearing  their  young  at  the  same  rate,  then  at 
the  end  of  seven  years  (a  short  life,  excluding  violent  deaths,  for 
any  bird)  there  will  be  2048  birds,  instead  of  the  original  sixteen. 
As  this  increase  is  quite  impossible,  we  must  conclude  either 
that  birds  do  not  rear  nearly  half  their  young,  or  that  the 
average  life  of  a  bird  is,  from  accident,  not  nearly  seven  years. 
Both  checks  probably  concur.  The  same  kind  of  calculation 
applied  to  all  plants  and  animals  affords  results  more  or  less 
striking,  but  in  very  few  instances  more  striking  than  in  man." 


"  Lighten  any  check  in  the  least  degree,  and  the  geometrical 
powers  of  increase  in  every  organism  will  almost  instantly 
increase  the  average  number  of  the  favoured  species.  .  .  . 
Finally,  let  it  be  borne  in  mind  that  this  average  number  of 
individuals  (the  external  conditions  remaining  the  same)  in  each 
country  is  kept  up  by  recurrent  struggles  against  other  species 
or  against  external  nature  (as  on  the  borders  of  the  Arctic 
regions,  where  the  cold  checks  life),  and  that  ordinarily  each 
individual  of  every  species  holds  its  place,  either  by  its  own 
struggle  and  capacity  of  acquiring  nourishment  in  some  period 
of  its  life,  from  the  egg  upwards;  or  by  the  struggle  of  its 
parents  (in  short-lived  organisms,  when  the  main  check  occurs 
at  longer  intervals)  with  other  individuals  of  the  same  or  different 
species. 

"  But  let  the  external  conditions  of  a  country  alter.  If  in  a 
small  degree,  the  relative  proportions  of  the  inhabitants  will  in 
most  cases  simply  be  slightly  changed ;  but  let  the  number  of 
inhabitants  be  small,  as  on  an  island,  and  free  access  to  it  from 
other  countries  be  circumscribed,  and  let  the  change  of  conditions 
continue  progressing  (forming  new  stations),  in  such  a  case  the 
original  inhabitants  must  cease  to  be  as  perfectly  adapted  to  the 


VIEWS   OF   CHAKLES  DAEWIN  387 

changed  conditions  as  they  were  originally.  It  has  been  shown 
in  a  former  part  of  this  work,  that  such  changes  of  external 
conditions  would,  from  their  acting  on  the  reproductive  system, 
probably  cause  the  organization  of  those  beings  which  were 
most  affected  to  become,  as  under  domestication,  plastic.  Now, 
can  it  be  doubted,  from  the  struggle  each  individual  has  to  obtain 
subsistence,  that  any  minute  variation  in  structure,  habits,  or 
instincts,  adapting  that  individual  better  to  the  new  conditions, 
^would  tell  upon  its  vigour  and  health  ?  In  the  struggle  it  would 
*  lave  a  better  chance  of  surviving ;  and  those  of  its  offspring 
which  inherited  the  variation,  be  it  ever  so  slight,  would  also 
have  a  better  chance.  Yearly  more  are  bred  than  can  survive; 
'the  smallest  grain  in  the  balance,  in  the  long  run,  must  tell  on 
which  death  shall  fall,  and  which  shall  survive.  Let  this  work 
of  selection  on  the  one  hand,  and  death  on  the  other,  go  on  for 
a  thousand  generations,  who  will  pretend  to  affirm  that  it  would 
produce  no  effect,  when  we  remember  what,  in  a  few  years, 
Bakewell  effected  in  cattle,  and  Western  in  sheep,  by  this  identical 
principle  of  selection  ?  " 

•  ••••• 

"  In  nature  we  have  some  slight  variation  occasionally  in  all 
parts ;  and  I  think  it  can  be  shewn  that  changed  conditions  of 
existence  is  the  main  cause  of  the  child  not  exactly  resembling 
its  parents ;  and  in  nature  geology  shews  us  what  changes  have 
taken  place,  and  are  taking  place.  We  have  almost  unlimited 

time ;  " 

•  •*•••• 

I"  Another  principle,  which  may  be  called  the  principle  of  diver- 
gence,  plays,  I  believe,  an  important  part  in  the  origin  of  species. 
The  same  spot  will  support  more  life  if  occupied  by  very  diverse 
forms.  We  see  this  in  the  many  generic  forms  in  a  square  yard 
of  turf,  and  in  the  plants  or  insects  on  any  little  uniform  islet, 
belonging  almost  invariably  to  as  many  genera  and  families 
as  species.  .  .  .  Now,  every  organic  being,  by  propagating  so 
raqidly,  may  be  said  to  be  striving  its  utmost  to  increase  in 
numbers.  So  it  will  be  with  the  offspring  of  any  species  after  it 
has  become  diversified  into  varieties,  or  subspecies,  or  true 
species.  And  it  follows,  I  think,  from  the  foregoing  facts,  that 
the  varying  offspring  of  each  species  will  try  (only  few  will 
succeed)  to  seize  on  as  many  and  as  diverse  places  in  the 
economy  of  nature  as  possible.  Each  new  variety  or  species, 
when  formed,  will  generally  take  the  place  of,  and  thus  exter- 
minate its  less  well-fitted  parent.  This  I  believe  to  be  the  origin 
of  the  classification  and  affinities  of  organic  beings  at  all  times  ; 
for  organic  beings  always  seem  to  branch  and  sub-branch  like 
the  limbs  of  a  tree  from  a  common  trunk,  the  flourishing  and 

c  c  2 


388        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

diverging  twigs  destroying  the  less  vigorous  —  the   dead  and  lost 
branches  rudely  representing  extinct  genera  and  families." 

The  same  paper  also  contains  a  sketch  of  the  supplementary 
theory  of  sexual  selection,  which  will  be  seen  to  agree  very  closely 
with  the  paragraph  from  Erasmus  Darwin's  "  Zoonomia  "  quoted 
in  the  last  chapter  :  — 

"  Besides  this  natural  means  of  selection,  by  which  those 
individuals  are  preserved,  whether  in  their  egg,  or  larval,  or 
mature  state,  which  are  best  adapted  to  the  place  they  fill  in 
nature,  there  is  a  second  agency  at  work  in  most  unisexual 
animals,  tending  to  produce  the  same  effect,  namely,  the  struggle 
of  the  males  for  the  females.  These  struggles  are  generally 
decided  by  the  law  of  battle,  but  in  the  case  of  birds,  apparently, 
by  the  charms  of  their  song,  by  their  beauty  or  their  power  of 
courtship,  as  in  the  dancing  rock-thrush  of  Guiana.  The  most 
vigorous  and  healthy  males,  implying  perfect  adaptation,  must 
generally  gain  the  victory  in  their  contests.  This  kind  of  selec- 
tion, however,  is  less  rigorous  than  the  other  ;  it  does  not  require 
the  death  of  the  less  successful,  but  gives  to  them  fewer 
descendants." 

The  publication  from  which  these  quotations  are  taken  itself 
consists  of  extracts  from  Charles  Darwin's  manuscripts,  selected 
with  a  view  to  explaining,  as  clearly  as  possible,  his  theory  of 
natural  selection,  and  is  therefore  especially  suitable  for  citation. 
In  the  following  year  (1859)  the  author's  classical  work,  "  The 
Origin  of  Species  by  means  of  Natural  Selection,"  made  its 
appearance,  the  first  of  that  notable  series  of  volumes  on 
philosophical  biology  which  have  made  his  name  so  famous. 
In  these  works  both  the  general  theory  of  Evolution  and  the  sub- 
sidiary theory  of  Natural  Selection  are  elaborated  and  supported 
by  an  immense  body  of  evidence  drawn  from  published  records 
and  personal  observations,  and  so  successfully  was  this  done  that 
in  a  comparatively  few  years  these  theories  met  with  general 
acceptance  on  the  part,  not  only  of  scientific  men,  but  also 
of  the  educated  public.  It  must  not  be  forgotten,  either,  that 
Charles  Darwin  applied  the  doctrine  of  organic  evolution 
in  a  fearless  and  uncompromising  manner  to  the  origin  of  the 
human  race. 


Dr.  Wallace's  contribution  to  the  Linnean  Society  symposium 
of  1858  was  entitled  "  On  the  Tendency  of  Varieties  to  depart 


VIEWS   OF   A.  E.  WALLACE  389 

indefinitely  from  the  Original  Type."  The  following  quotations 
will  serve  to  show  that  his  views  on  Natural  Selection  were  closely 
similar  to  those  of  Charles  Darwin. 

"  The  life  of  wild  animals  is  a  struggle  for  existence.  The 
full  exertion  of  all  their  faculties  and  all  their  energies  is 
required  to  preserve  their  own  existence  and  provide  for  that  of 
their  infant  offspring.  The  possibility  of  procuring  food  during 
the  least  favourable  seasons,  and  of  escaping  the  attacks  of  their 
most  dangerous  enemies,  are  the  primary  conditions  which 
determine  the  existence  both  of  individuals  and  of  entire 
species." 

"  Even  the  least  prolific  of  animals  would  increase  rapidly  if 
unchecked,  whereas  it  is  evident  that  the  animal  population  of 
the  globe  must  be  stationary,  or  perhaps,  through  the  influence 
of  man,  decreasing.  Fluctuations  there  may  be ;  but  permanent 
increase,  except  in  restricted  localities,  is  almost  impossible. 
For  example,  our  own  observation  must  convince  us  that  birds 
do  not  .go  on  increasing  every  year  in  a  geometrical  ratio,  as 
they  would  do,  were  there  not  some  powerful  check  to  their 
natural  increase.  Very  few  birds  produce  less  than  two  young 
ones  each  year,  while  many  have  six,  eight,  or  ten ;  four  will 
certainly  be  below  the  average ;  and  if  we  suppose  that  each  pair 
produce  young  only  four  times  in  their  life,  that  will  also  be  below 
the  average,  supposing  them  not  to  die  either  by  violence  or  want 
of  food.  Yet  at  this  rate  how  tremendous  would  be  the  increase 
in  a  few  years  from  a  single  pair  !  A  simple  calculation  will 
show  that  in  fifteen  years  each  pair  of  birds  would  have  increased 
to  nearly  ten  millions  !  Whereas  we  have  no  reason  to  believe 
that  the  number  of  the  birds  of  any  country  increases  at  all  in 
fifteen  or  in  one  hundred  and  fifty  years.  With  such  powers  of 
increase  the  population  must  have  reached  its  limits,  and  have 
become  stationary,  in  a  very  few  years  after  the  origin  of  each 
species.  It  is  evident,  therefore,  that  each  year  an  immense 
number  of  birds  must  perish — as  many  in  fact  as  are  born.  .  .  . 
It  is,  as  we  commenced  by  remarking, '  a  struggle  for  existence,' 
in  which  the  weakest  and  least  perfectly  organized  must  always 
succumb. 

"  Now  it  is  clear  that  what  takes  place  among  the  individuals 
of  a  species  must  also  occur  among  the  several  allied  species  of 
a  group, — viz.,  that  those  which  are  best  adapted  to  obtain  a 
regular  supply  of  food,  and  to  defend  themselves  against  the 
attacks  of  their  enemies  and  the  vicissitudes  of  the  seasons,  must 
necessarily  obtain  and  preserve  a  superiority  in  population ; 
while  those  species  which  from  some  defect  of  power  or  organization 
are  the  least  capable  of  counteracting  the  vicissitudes  of  food 


390        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

supply,  &c.,  must  diminish  in  numbers,  and,  in  extreme  cases, 
become  altogether  extinct." 

< 

"  Most  or  perhaps  all  the  variations  from  the  typieal  form  of 
a  species  must  have  some  definite  effect,  however  slight,  on  the 
habits  or  capacities  of  the  individuals.  Even  a  change  of  colour 
might,  by  rendering  them  more  or  less  distinguishable,  affect 
their  safety ;  a  greater  or  less  development  of  hair  might  modify 
their  habits.  .  .  .  An  antelope  with  shorter  or  weaker  legs  must 
necessarily  suffer  more  from  the  attacks  of  the  feline  carnivora. 
...  If,  on  the  other  hand,  any  species  should  produce  a  variety 
having  slightly  increased  powers  of  preserving  existence,  that 
variety  must  inevitably  in  time  acquire  a  superiority  in  numbers. 
...  All  varieties  will  therefore  fall  into  two  classes — those 
which  under  the  same  conditions  would  never  reach  the  popula- 
tion of  the  parent  species,  and  those  which  would  in  time  obtain 
and  keep  a  numerical  superiority.  Now,  let  some  alteration 
of  physical  conditions  occur  in  the  district — a  long  period  of 
drought,  a  destruction  of  vegetation  by  locusts,  the  irruption  of 
some  new  carnivorous  animal  seeking  '  pastures  new ' — any 
change  in  fact  tending  to  render  existence  more  difficult  to  the 
species  in  question,  and  tasking  its  utmost  powers  to  avoid  com- 
plete extermination ;  it  is  evident  that,  of  all  the  individuals 
composing  the  species,  those  forming  the  least  numerous  and 
most  feebly  organized  variety  would  suffer  first,  and,  were  the 
pressure  severe,  must  soon  become  extinct.  The  same  causes 
continuing  in  action,  the  parent  species  would  next  suffer, 
would  gradually  diminish  in  numbers,  and  with  a  recurrence 
of  similar  unfavourable  conditions  might  also  become 
extinct.  The  superior  variety  would  then  alone  remain,  and 
on  a  return  to  favourable  conditions  would  rapidly  increase 
in  numbers  and  occupy  the  place  of  the  extinct  species  and 
variety. 

"  The  variety  would  now  have  replaced  the  species,  of  which  it 
would  be  a  more  perfectly  developed  and  more  highly  organized 
form.  It  would  be  in  all  respects  better  adapted  to  secure  its 
safety,  and  to  prolong  its  individual  existence  and  that  of  the 
race.  Such  a  variety  could  not  return  to  the  original  form ;  for 
that  form  is  an  inferior  one,  and  could  never  compete  with  it 
for  existence.  .  .  .  But  this  new,  improved,  and  populous  race 
might  itself,  in  course  of  time,  give  rise  to  new  varieties, 
exhibiting  several  diverging  modifications  of  form,  any  of  which, 
tending  to  increase  the  facilities  for  preserving  existence,  must, 
by  the  same  general  law,  in  their  turn  become  predominant. 
Here,  then,  we  have  progression  and  continued  divergence  deduced 
from  the  general  laws  which  regulate  the  existence  of  animals  in 


CHAKLES   DARWIN   AND   LAMARCK  391 

a  state  of  nature,  and  from  the  undisputed  fact  that  varieties  do 
frequently  occur." 

It  will  be  evident  from  the  above  sketch  of  the  theory  of 
Natural  Selection,  which  I  have  thought  it  desirable  to  give  in 
the  actual  words  of  its  chief  exponents,  that  adaptation  is 
explained  as  the  logical  consequence  of  certain  facts  which  can 
at  any  time  be  verified  by  direct  observation.  (1)  All 
organisms  tend  to  increase  in  a  high  geometrical  ratio ;  (2)  there 
is,  partly  as  a  direct  result  of  such  increase,  a  keen  struggle  for 
existence,  to  which  all  organisms  are  more  or  less  exposed  and  in 
which  vast  numbers  perish  without  leaving  offspring;  (3)  all 
organisms  tend  to  vary  in  many  directions ;  (4)  variations, 
whether  favourable  or  otherwise,  tend  to  be  transmitted  by 
heredity  from  generation  to  generation;  though,  as  we  have 
already  seen,  there  is  at  the  present  time  much  dispute  as  to 
whether  variations  of  a  certain  kind  ought  not  to  be  excluded 
from  this  generalization. 

It  follows  inevitably  from  these  premisses  that  in  every  genera- 
tion there  will  be  a  more  or  less  strongly  pronounced  tendency 
towards  the  elimination  of  those  individuals  which  are  least  well 
adapted  to  their  environment  and  a  corresponding  preservation 
and  encouragement  of  those  which  are  best  adapted,  or,  in 
Herbert  Spencer's  celebrated  phrase,  a  "  survival  of  the  fittest." 
This  process,  continued  from  generation  to  generation  for  count- 
less ages,  has  resulted  in  that  marvellous  perfection  of  adaptation 
which  we  have  seen  to  be  such  a  striking  feature  of  both  plants 
and  animals. 

Charles  Darwin  himself,  however,  was  not  satisfied  with  natural 
selection  as  the  sole  factor  concerned  in  bringing  about  pro- 
gressive evolution  and  adaptation.  Although,  in  the  historical 
sketch  which  he  added  to  the  later  editions  of  the  "  Origin  of 
Species,"  he  remarks  : — 

"It  is  curious  how  largely  my  grandfather,  Dr.  Erasmus 
Darwin,  anticipated  the  views  and  erroneous  grounds  of  opinion 
of  Lamarck  in  his  '  Zoonomia,' ' 

and  although  he  himself  at  first  appears  to  have  attached  very 
little  importance  to  Lamarck's  opinions,  yet  we  find  in  the  last 
chapter  of  the  sixth  edition  of  the  "  Origin  of  Species  "  abundant 
evidence  that  he  was  obliged  to  admit  the  efficacy  of  the  chief 
"  Lamarckian "  factor,  the  principle  of  use  and  disuse,  in 


392        OUTLINES   OF  EVOLUTIONAEY  BIOLOGY 

modifying  species,  and  also,  to  some  extent,  that  of  the  direct 
action  of  the  environment : — 

"  Disuse,  aided  sometimes  by  natural  selection,  will  often  have 
reduced  organs  when  rendered  useless  under  changed  habits  or 
conditions  of  life;  and  we  can  understand  on  this  view  the 
meaning  of  rudimentary  organs.1  But  disuse  and  selection  will 
generally  act  on  each  creature,  when  it  has  come  to  maturity 
and  has  to  play  its  full  part  in  the  struggle  for  existence,  and 
will  thus  have  little  power  on  an  organ  during  early  life  ;  hence 
the  organ  will  not  be  reduced  or  rendered  rudimentary  at  this 
early  age.  The  calf,  for  instance,  has  inherited  teeth,  which 
never  cut  through  the  gums  of  the  upper  jaw,  from  an  early  pro- 
genitor having  well-developed  teeth ;  and  we  may  believe,  that 
the  teeth  in  the  mature  animal  were  formerly  reduced  by  disuse, 
owing  to  the  tongue  and  palate,  or  lips,  having  become 
excellently  fitted  through  natural  selection  to  browse  without  their 
aid ;  whereas  in  the  calf,  the  teeth  have  been  left  unaffected,  and 
on  the  principle  of  inheritance  at  corresponding  ages  have  been 
inherited  from  a  remote  period  to  the  present  day."  2 


"  I  have  now  recapitulated  the  facts  and  considerations  which 
have  thoroughly  convinced  me  that  species  have  been  modified, 
during  a  long  course  of  descent.  This  has  been  effected  chiefly 
through  the  natural  selection  of  numerous  successive,  slight, 
favourable  variations  ;  aided  in  an  important  manner  by  the 
inherited  effects  of  the  use  and  disuse  of  parts  ;  and  in  an 
unimportant  manner,  that  is  in  relation  to  adaptive  structures, 
whether  past  or  present,  by  the  direct  action  of  external  condi- 
tions, and  by  variations  which  seem  to  us  in  our  ignorance  to 
arise  spontaneously.  It  appears  that  I  formerly  underrated  the 
frequency  and  value  of  these  latter  forms  of  variation,  as  leading 
to  permanent  modifications  of  structure  independently  of  natural 
selection.  But  as  my  conclusions  have  lately  been  much  mis- 
represented, and  it  has  been  stated  that  I  attribute  the  modifica- 
tion of  species  exclusively  to  natural  selection,  I  may  be  permitted 
to  remark  that  in  the  first  edition  of  this  work,  and  subsequently, 
I  placed  in  a  most  conspicuous  position — namely,  at  the  close  of 
the  Introduction — the  following  words  :  '  I  am  convinced  that 
natural  selection  has  been  the  main  but  not  the  exclusive  means 
of  modification.'  This  has  been  of  no  avail.  Great  is  the 
power  of  steady  misrepresentation  ;  but  the  history  of  science 
shows  that  fortunately  this  power  does  not  long  endure."  3 

*  Often  now  called  "  vestigial  organs." 
2  "  Origin  of  Species,"  Ed.  vi.  p.  420. 
8  Ibid.,  p.  421. 


A.  B.  WALLACE  AND  LAMAECK       393 

It  is  of  the  greatest  interest  to  recognize  the  fact  that  Darwin 
himself  saw  nothing  incompatible  between  the  so-called 
Lamarckian  factors  of  use  and  disuse  and  the  direct  action  of 
the  environment,  and  the  principle  of  natural  selection,  but,  on 
the  other  hand,  that  the  one  set  of  factors  might  supplement  the 
other. 

On  the  occasion  of  the  unveiling  of  the  statue  of  Charles 
Darwin  in  the  Natural  History  Museum  at  South  Kensington, 
Professor  Huxley  found  occasion  to  observe  that  "  science 
commits  suicide  when  it  adopts  a  creed."1  This  warning,  it  is 
to  be  feared,  has  not  been  heeded  by  all  of  Darwin's  followers. 
Many  of  these  have  departed  very  far  from  the  moderate  and 
rational  position  of  their  leader  and,  while  attributing  to  natural 
selection  almost  every  advance  which  has  been  made  in  the 
evolution  of  the  organic  world,  are,  as  we  have  already  seen, 
obliged  to  justify  their  neglect  of  the  "  Lamarckian  "  factors  by 
denying  altogether  the  possibility  of  the  inheritance  of  acquired 
characters,  which  Darwin,  of  course,  freely  admitted.  Natural 
selection,  in  the  hands  of  these  enthusiasts,  and  in  spite  of 
Charles  Darwin's  efforts  to  maintain  a  just  balance  between  this 
and  other  factors,  has  indeed  become  a  creed. 

Dr.  Wallace  from  the  first  adopted  an  uncompromising 
attitude  towards  the  opinions  of  Lamarck.  In  the  Linnean 
Society  paper  from  which  we  have  already  quoted  he  says  :— 

"  The  hypothesis  of  Lamarck — that  progressive  changes  in 
species  have  been  produced  by  the  attempts  of  animals  to  increase 
the  development  of  their  own  organs,  and  thus  modify,  their 
structure  and  habits — has  been  repeatedly  and  easily  refuted  by  all 
writers  on  the  subject  of  varieties  and  species,  and  it  seems  to  have 
been  considered  that  when  this  was  done  the  whole  question  has 
been  finally  settled ;  but  the  view  here  developed  renders  such 
an  hypothesis  quite  unnecessary,  by  shewing  that  similar  results 
must  be  produced  by  the  action  of  principles  constantly  at  work 
in  nature.  The  powerful  retractile  talons  of  the  falcon-  and  the 
cat-tribes  have  not  been  produced  or  increased  by  the  volition 
of  those  animals ; 2  but  among  the  different  varieties  which 
occurred  in  the  earlier  and' less  highly  organized  forms  of  these 
groups,  those  ahvays  survived  longest  which  had  the  greatest  facilities 
or  seizing  their  prey" 

1  Vide  Herbert  Spencer's  "  Factors  of  Organic  Evolution,"  p.  75. 

2  Who  ever  said  they  had,  except  in  the  sense  that  an  animal  voluntarily  uses  its 
slaws  on  appropriate  occasions  and  that  constantly  repeated  use  causes  them  to 


394        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

Wallace's  views l  also  differ  from  those  of  Darwin  in  that,  at 
any  rate  in  later  years,  they  have  become  strongly  anthropo- 
centric,  and  he  now  regards  the  whole  of  the  organic  world  as 
having  been  designed  by  the  Creator  for  the  ultimate  reception 
and  benefit  of  mankind.  He  does  not,  it  is  true!  go  back  to  the 
old  idea  that  species  have  been  separately  and  specially  created 
as  we  now  find  them,  but  he  holds  that  the  el  tire  scheme  of 
evolution  was  planned  out  in  the  mind  of  the  Cpator,  and  even 
suggests  that  the  working  out  of  this  scheme  may  have  been 
delegated  by  the  Supreme  Being  to  a  body  of  "organizing 
spirits  "  : — 

A.-" 

"  At  successive  stages  of  development  of  the  life-world,  more 
and  perhaps  higher  intelligences  might  be  required  to  direct  the 
main  lines  of  variation  in  definite  directions  in  accordance  with 
the  general  design  to  be  worked  out,  and  to  guard  against  a 
break  in  the  particular  line  which  alone  could  lead  ultimately  to 
the  production  of  the  human  form."  2  •< ' 

Such  speculations  as  this  would  render  natural  selection -and 
all  other  natural  factors  of  organic  evolution  superfluous,  but  we 
cannot  profitably  discuss  them  in  a  work  like  the  present.  We 
may  point  out,  however,  that  they  are  in  essential  agreement  with 
the  views  of  the  author  of  the  "  Vestiges  of  Creation,"  to  which 
we  have  referred  in  the  early  part  of  this  chapter,  excepting  tjailt 
Roberi^Chambers  did  not  venture  to  cafl  in  the  assistants  of 
subordinate  "  organizing  spirits"  to  carry  out  the  plans  oJ^be 
Creator. 

A 

increase  in  size  and  efficiency  ?     Lamarck  did  not  suppose  that  an  animal  simply 
willed  organs  to  sprout  out  of  its  body  ! 

1  For  a  full  exposition  of  these  views  the  reader  should  refer  to  Dr.  Wallace's 
"  Darwinism  "  (London  :  Macmillan  &  Co.,  1889). 

2  "  The  World  of  Life,  a  Manifestation  of  Creative  Power,  Directive  Mind  and 
Ultimate  Purpose,"  by  Alfred  Russel  Wallace  (London  :  Chapman  and  Hall,  Ltd., 
1910),  p.  395. 


CHAPTER  XXVI 

Selection  not  confined  to  the  organic  world — Illustrations  of  the  action  of 
natural  selection  in  the  struggle  for  existence — Degeneration — Flight- 
less birds — Extermination  of  the  Morioris — Sedentary  animals — Para- 
sites— Co-operation  of  natural  selection  and  the  so-called  Lamarckian 
factors  of  evolution — The  influence  of  internal  secretions  upon  growth 
— Increase  in  size  beyond  the  limits  of  utility. 

THE  principle  of  selection  is,  of  course,  by  no  means  confined 
to  living  things.  The  various  bodies  which  make  up  the 
inorganic  world  owe  their  actual  form  and  arrangement  largely 
to  processes  of  selection  which  are  constantly  going  on  amongst 
them.  The  outline  of  the  sea  coast  is  the  result  of  the  selective 
action  of  atmospheric  and  tidal  agencies  upon  the  different  kinds 
of  rock  of  which  it  is  composed.  The  softer  parts  are  destroyed 
first,  leaving  the  more  resistant  portions  to  stand  out  in  the  form 
of  bluffs  or  promontories,  and  to  illustrate  in  the  inanimate 
world  the  principle  of  the  survival  of  the  fittest.  We  might  even 
say  that  the  prominent  headlands  exhibit  adaptation,  for  if  they 
were  not  adapted  by  their  peculiar  hardness  to  resist  the  dis- 
integrating influences  of  the  environment  they  would  not  be 
there,  but  would  have  perished  with  those  portions  of  the  land 
which  formerly  occupied  the  bays  and  inlets. 

All  things,  in  short,  must  be  subject  to  the  selective  action  of 
their  environment,  and  we  need  not  hesitate  to  attribute  to 
natural  selection  a  very  large  share  in  the  modelling  of  the 
features  of  the  organic  world  as  we  now  see  it.  We  know  what 
we  ourselves,  by  our  so-called  artificial  selection,  are  able  to  do 
in  this  way.  The  chief  difference  between  artificial  and  natural 
selection  is  that  man  selects  for  his  own  purposes  and  modifies 
organisms  to  suit  his  own  ends,  while  Nature  selects  to  the  benefit 
of  the  species  operated  upon,  which  becomes  thereby  modified  to 
its  own  advantage  and  preservation  in  the  struggle  for  existence. 
But  we  cannot  really  draw  a  distinction  between  the  two  kinds 
of  selection,  for  even  in  a  state  of  nature  organisms  are  often 
selected  and  modified  to  the  advantage  of  other  organisms. 


396        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

As  we  saw  in  a  previous  chapter,  the  forms,  colours  and  scents 
of  many  flowers  are  probably  the  result  of  unconscious  selection 
by  insects,  extending  over  countless  generations.  It  may  be  said 
that  the  advantage  gained  in  this  case  is  mutual ;  the  insect  gets 
the  honey  and  the  flower  gets  fertilized.  This  of  course  is  true, 
but  exactly  the  same  is  true  of  human  selection.  The  sheep  gets 
the  pasture  and  man  gets  the  wool. 

'  It  seems  impossible  to  explain  on  any  other  hypothesis  than 
that  of  the  natural  selection  and  gradual  accumulation  of  chance, 
favourable  variations,  those  marvellous  adaptations  of  animals 
which  lead  to  protective  resemblance  and  mimicry,  for  although 
we  may  admit  that  an  organ  which  is  actively  employed  may  be 
modified  by  the  efforts  of  an  animal  to  maintain  itself  by  the 
use  of  that  organ,  we  can  hardly  extend  the  same  principle  to 
such  passive  features  as  colour  and  ornamentation,  or  the  out- 
growth of  leaf-like  dermal  appendages  and  so  forth.  It  may  be 
questioned  if,  even  with  the  aid  of  natural  selection,  we  can  fully 
account  for  all  the  wonderful  phenomena  of  mimicry,  for  why,  if 
it  be  an  advantage  to  some  species  to  adopt  a  common  warning 
colour  and  band  themselves  together  in  synaposematic  groups, 
should  it  be  desirable  for  others  to  do  just  the  reverse  and  split 
up  into  a  number  of  differently  coloured  forms,  each  of  which 
mimics  some  particular  model  ?  We  can  only  say  that  we  do 
not  know  all  the  factors  of  the  environment,  and  that  until  we  do 
our  inability  to  solve  the  problem  cannot  be  justly  considered  as 
an  argument  against  the  efficacy  of  natural  selection. 

•  It  is,  of  course,  extremely  difficult,  if  not  impossible,  to  obtain 
direct  evidence  of  the  action  of  natural  selection  in  modifying 
species  in  a  state  of  nature.  Human  life  is  all  too  brief  to 
admit  of  our  making  very  satisfactory  observations  concerning 
processes  which  extend  perhaps  over  millions  of  years.  Man 
has,  however,  in  a  comparatively  short  space  of  time,  so  changed 
the  conditions  of  life  for  many  of  the  lower  animals  as  to  lead, 
albeit  unintentionally,  to  the  more  or  less  complete  extermina- 
tion of  many  species,  and  by  studying  these  cases  we  may  hope 
to  arrive  at  sound  conclusions  as  to  what  takes  place  in  a  state  of 
nature.  After  all,  mankind  is  a  part  of  nature  and  we  have  no 
just  reason  for  excluding  his  influence  in  our  consideration  of 
the  factors  which  have  brought  about  the  present  condition  of 
the  organic  world. 

It  is  well  known  that  many  of  the   birds    of  various  remote 


FLIGHTLESS   BIEDS  397 

islands  have  lost  the  power  of  flight.  Such  are  the  kiwi,  the 
kakapo,  the  weka,  the  notornis  and  the  already  extinct  gigantic 
moas  of  New  Zealand  ;  the  dodo  of  Mauritius,  and  the  solitaire 
of  Eodriguez.  Although  belonging  to  several  very  distinct 
families  of  birds,  including  ratites,  parrots,  rails  and  pigeons,  all 
the  forms  enumerated  have  undergone  the  same  curious  modifi- 
cation, resulting  in  the  most  extreme  cases  (the  moas)  in  the 
complete  loss  of  the  wings,  and  in  others  in  the  reduction  of  those 
organs  to  a  more  or  less  vestigial  condition.1 

This  convergence  is  clearly  due  to  the  similarity  of  the  con- 
ditions under  which  these  birds  have  had  to  live.  One  of  the 
most  characteristic  features  of  oceanic  islands  is  the  absence 
from  them  of  predaceous  mammals,  the  natural  enemies  of  birds, 
which  have  never  been  able  to  cross  the  great  stretches  of  open 
ocean  which  separate  such  islands  from  the  continental  areas  on 
which  the  Mammalia  have  been  evolved.  Birds,  however,  and 
even  land  birds,  by  virtue  of  their  powers  of  flight,  have  been 
able  to  reach  these  islands  at  more  or  less  frequent  intervals 
and  to  establish  themselves  there.  Finding  abundance  of  food, 
which  they  could  obtain  near  the  ground,  and  finding  themselves 
no  longer  under  the  necessity  of  constantly  using  their  wings  in 
order  to  escape  from  their  enemies,  some  of  these  birds,  though 
by  no  means  all,  gradually  gave  up  flying  and  their  wings  under- 
went a  slow  process  of  degeneration  in  accordance  with  Lamarck's 
principle  of  disuse.  No  doubt  such  disuse.,  if  continued  only 
through  a  single  lifetime,  could  scarcely  produce  a  visible  effect 
upon  the  next  generation,  but  continued  under  the  same  con- 
ditions throughout  thousands  of  generations  it  has  brought  about 
a  permanent  deterioration  which  can  no  longer  be  retrieved. 

It  is  to  be  noted  that  this  degeneration  is  the  result  of  the 
removal  of  the  organism,  to  a  certain  extent,  from  the  struggle 
for  existence.  Na.tnra.1  ^election  can  only  ^  frhvnnnrh  f,hp  fltTiipHp 
those,  organs  whigji  are  of  value  in  jhe 


^truggle.  When  the  struggle  ceases,  natural  selection  ceases  and 
Regeneration  sets  in,  for  there  is  no  longer  any  reason  why  a 
high  standard  of  perfection  should  be  maintained.  All  degrees  of 
imperfection  now  have  equal  opportunities  of  propagating  them- 
selves. The  inferior  individuals  are  no  longer  weeded  out,  and 
the  average  condition  of  the  species  consequently  deteriorates. 
But  observe  what  happens  when  a  degenerate  organism  is 

1  Compare  Chapter  XVII,  Figs.  Ill,  112. 


OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

once  more  exposed,  by  some  unfortunate  change  in  its  environ- 
ment, to  the  old  struggle  from  which  it  had  escaped.  This  has 
actually  taken  place  in  the  case  of  the  flightless  birds  of  New 
Zealand  and  other  remote  islands.  With  the  advent  of  Euro- 
peans, predaceous  mammals  of  many  species — dogs,  cats,  rats, 
weasels,  stoats  and  ferrets — have  been  let  loose  upon  their 
helpless  victims.  These  are  once  more  exposed  to  a  keen 
struggle  for  existence,  while  at  the  same  time  they  have  lost 
those  very  organs  which  are  necessary  to  enable  them  to  maintain 
themselves  in  that  struggle,  and  natural  selection,  having 
regained  her  power,  is  rapidly  exterminating  them.  It  is  not 
too  much  to  say  that  in  a  few  years'  time  there  will  be  no 
flightless  birds  left  in  New  Zealand  except  in  special  reserves 
where  they  are  being  protected  by  man. 

It  is  highly  instructive  in  this  connection  to  contrast  the  con- 
dition of  such  a  bird  as  the  flightless  parrot,  or  kakapo,  with  that 
of  its  relative  the  kea.  The  kakapo  is  a  large,  heavy  bird  of 
nocturnal  habits  and  with  practically  no  means  of  defence ;  it 
haunts  the  dense  forest  and  is  rarely  seen  except  when  hunted 
out  by  dogs.  The  kea,  on  the  contrary,  is  one  of  the  strongest 
fliers  of  the  parrot  tribe.  It  frequents  high  and  more  or  less 
inaccessible  mountain  regions  and  since  the  advent  of  Europeans 
has  learnt  to  make  use  of  the  sheep  which  they  have  introduced 
as  an  additional  food  supply.  It  is  doubtful  whether  the  utmost 
efforts  of  the  sheep  farmers,  Who  annually  expend  large  sums  of 
money  for  the  purpose,  will  ever  enable  them  to  exterminate  the 
kea,  and  it  is  equally  doubtful  whether  the  efforts  of  the  New 
Zealand  Government  to  preserve  the  unique  flightless  birds  will 
sufiice  to  prevent  the  complete  extermination  of  the  kakapo 
within  the  next  few  years. 

The  aboriginal  human  population  of  remote  islands  has  of 
course  suffered  not  less  than  the  lower  animals  from  the  in- 
vasion of  their  retreats  by  Europeans,  although  not  always 
exclusively  at  the  hands  of  the  Europeans  themselves.  There 
are,  perhaps,  few  more  striking  examples  of  the  extermination  of 
a  primitive  native  race  than  that  afforded  by  the  rapid  disap- 
pearance of  the  Moriori  inhabitants  of  the  Chatham  Islands, 
some  four  hundred  miles  to  the  east  of  New  Zealand,  during  the 
nineteenth  century.1  At  the  time  of  my  visit  to  these  islands,  in 

1  Compare  Dendy, "  The  Chatham  Islands  :  A  Study  in  .biology  "  (Memoirs  and  Pro- 
ceedings of  the  Manchester  Literary  and  Philosophical  Society,  Vol.  XLVL,  1902). 


EXTEKMINATION   OF   MOKIOEIS  399 

January,  1901,  there  were  only  about  a  dozen  pure-blooded  indi- 
viduals left ;  some  of  these  were  of  great  age,  while  the  youngest 
was  a  lad  of  about  16,  and  they  had  all,  I  think,  more  or  less 
completely  adopted  European  manners  and  customs.  Under 
these  circumstances  we  are  fortunate  in  possessing  any  reliable 
record  of  this  interesting  people,  and  that  we  do  so  is  largely 
due  to  the  energy  and  enthusiasm  of  Mr.  Alexander  Shand,  who 
for  more  than  thirty  years  lived  amongst  the  Morioris  and 
made  a  special  study  both  of  that  race  and  of  their  Maori 
conquerors. 

It  appears  from  their  language,  customs  and  traditions,  as 
well  as  from  their  physical  characteristics,  that  the  Morioris  are 
closely  related  to  the  New  Zealand  Maoris.  Their  ignorance  of 
the  art  of  tattooing,  and  their  very  inferior  artistic  faculties  in 
general,  however,  point  to  a  very  remote  separation  of  the  two 
races. 

Like  the  Maoris  they  trace  their  origin  to  an  unknown  father- 
land called  Hawaiki,  from  which  they  must  have  emigrated  to 
Chatham  Island  in  canoes.  In  their  new  home  they  appear  to 
have  found  the  conditions  of  life  remarkably  easy,  indeed,  as  the 
sequel  shows,  fatally  so.  With  an  abundant  natural  food  supply 
of  fruit,  shell-fish,  &c.,  and  with  no  enemies  to  contend  with, 
they  multiplied  until  the  islands  were  thickly  populated,  while 
at  the  same  time  they  doubtless  became  lazy  and  effeminate. 

The  discovery  of  the  islands  by  the  brig  "  Chatham,"  in  1790, 
may  be  said  to  have  sealed  the  fate  of  the  unfortunate  Moriori, 
though  it  is  doubtful  whether  any  serious  injury  ensued  until 
the  advent  of  the  whaling  and  sealing  vessels  in  1828.  These 
vessels  brought  with  them  many  undesirable  visitors,  and  prob- 
ably were  the  means  of  introducing  a  disease  which  soon  played 
havoc  with  the  native  race.  On  board  some  of  the  ships,  moreover, 
were  Maoris  from  New  Zealand,  who,  on  their  return,  painted 
such  a  glowing  picture  of  the  land  of  plenty,  that  a  large  number 
of  their  fellow-countrymen  determined  to  emigrate  to  the  islands 
en  masse. 

In  order  to  effect  this  purpose  they  took  possession  of  the  brig 
"  Rodney "  at  Port  Nicholson,  in  New  Zealand,  about  the 
beginning  of  November,  1835.  They  are  said  to  have  seized  the 
crew  and -compelled  the  captain  to  transport  them,  about  900  in 
number,  to  their  destination.  At  the  time  of  the  invasion  the 
Morioris  are  supposed  to  have  numbered  about  2000,  and  had  they 


400        OUTLINES   OF   EVOLUTIONABY  BIOLOGY 

attacked  the  new-comers  on  their  first  arrival,  they  might  have 
exterminated  them  with  little  trouble  and  prolonged  for  an 
indefinite  period  the  life  of  their  own  race.  Unfortunately  for 
themselves,  however,  they  had  lost  the  art  of  self-defence.  Owing 
to  the  absence  of  competition  they  had,  in  this  respect  at  any 
rate,  undergone  degeneration.  Killing  was  actually  forbidden 
by  their  laws,  and  peace  had  reigned  too  long  and  too  securely  to 
give  place  at  once  to  war  when  the  emergency  arose. 

Just  as  the  flightless  birds  of  New  Zealand  have  more  or  less 
/completely  disappeared  since  the  advent  of  carnivorous  mammals, 
so  the  Morioris,  their  happy  isolation  once  broken,  fell  an  easy 
prey  to  the  more  virile  Maoris.  The  latter  proceeded  to  parcel 
out  the  conquered  country  amongst  themselves,  claiming  not 
only  the  land  but  also  the  inhabitants  thereof,  many  of  whom 
were  massacred  under  circumstances  of  unutterable  atrocity, 
while  the  remnant  were  speedily  reduced  to  the  condition  of 
slaves.  Under  the  changed  conditions  which  had  suddenly  arisen 
in  their  environment  the  Morioris  were  no  longer  fit  to  survive 
in  the  struggle  for  existence,  they  had  become  degenerate  in  a 
vital  respect,  and  natural  selection,  as  soon  as  opportunity  arose, 
stepped  in  and  eliminated  them. 

It  would  be  easy  to  multiply  illustrations  of  the  great  generaliza- 
tion that  when  removed  from  the  struggle  for  existence  all 
organisms  tend  to  become  degenerate,  the  organs  or  faculties 
which  they  no  longer  require  atrophying  and  gradually  dis- 
appearing for  want  of  employment.  We  see  this  very  clearly  in 
the  case  of  sedentary  animals  such  as  the  ascidians  (Figs.  129, 
130).  The  young  ascidian  is  a  highly  organized  creature  which 
swims  actively  about  by  means  of  a  muscular  tail,  in  the  same 
way  as  the  tadpole  of  a  frog.  Like  the  latter  it  has  nervous 
system,  notochord  and  sense  organs — though  the  sense  organs 
are  of  a  type  peculiar  to  itself — and  is  an  undoubted  chordate. 
It  never,  however,  progresses  further  in  organization,  so  as  to 
attain  the  true  vertebrate  condition.  On  the  contrary,  it  gives  up 
its  active  life  and  withdraws  as  far  as  possible  from  the  struggle 
for  existence  by  fixing  itself  to  some  rock  or  seaweed  and  envelop- 
ing its  entire  body  in  a  thick  protective  envelope,  within  which 
it  undergoes  extensive  degeneration.  The  tail  and  notochord 
completely  disappear,  so  do  the  sense  organs,  none  of  these 
being  any  longer  required  under  the  new  conditions  of  life.  The 


DEGENERATION   IN   PARASITES 


401 


nervous  system  dwindles  away  to  a  mere  ganglion,  from  which  a 
few  nerves  come  off,  and  the  entire  animal  is  reduced  to  the 
condition  of  a  bag,  with  two  openings  through  which  the 
remaining  organs  obtain  their  food  supply  and  communicate 
with  the  outside  world  by  means  of  a  stream  of  water  maintained 
by  ciliary  action. 

Still  more  conspicuous  is  the  degeneration  undergone  by 
the  great  majority  of  parasites,  whether  animals  or  plants. 
Sacculina,  for  example,  in  the  earlier  stages  of  its  existence,  is 
an  active  crustacean  which  swims  vigorously  about  by  means  of 
well  developed  appen- 
dages. It  belongs  to  a 
group,  the  barnacles  or 
cirripedes,  which  are 
notorious  for  sedentary 
habits  and  consequent 
degeneration  in  the  adult 
condition.  Sacculina, 
however,  not  content  with 
a  sedentary  life,  goes 
further  down  hill  and 
becomes  parasitic.  It 
attacks  crabs,  and  in  the 
adult  state  is  reduced  to 
the  condition  of  a  large, 
irregularly  shaped  bag 
(Fig.  183)  fixed  to  the 
under  surface  of  the  crab's 
abdomen  by  root -like  pro- 
cesses which  penetrate  the  body  of  the  host  and  extract  nutri- 
ment therefrom.  With  the  exception  of  these  root-like  processes, 
which  are  a  special,  caenogenetic  development,  adapted  for  nutri- 
tion under  new  conditions  of  life,  the  only  organs  which  have 
not  undergone  degeneration  are  those  of  reproduction,  for  upon 
these  depends  the  perpetuation  of  the  race  and  upon  these, 
therefore,  natural  selection  is  still  able  to  retain  her  hold. 

It  is,  indeed,  a  general  rule  amongst  parasitic  animals  that  the 
reproductive  organs  are  largely  developed  and  very  complicated, 
for  the  conditions  which  have  become  necessary  for  the  existence 
of  these  animals  are  so  complex  and  highly  specialized,  while 
the  chances  of  mating  between  different  individuals  for  purposes 


FIG.  183. — Lower  surface  of  a  Swimming 
Crab  (Portunus  depurator)  with  a  Sac- 
culina (Sac.)  attached  to  it.  (From  a 
photograph.) 


B. 


D    P 


402        OUTLINES    OF   EVOLUTIONARY  BIOLOGY 

of  sexual  reproduction  are  so  remote,  that  the  ova  and  spermatozoa 
have  to  be  produced  in  vast  numbers  to  compensate  for  the 
immense  mortality  which  must  take  place  amongst  them  and 
amongst  the  young  animals  to  which  they  may  give  rise.  Thus 


ffiG.  184. — The  Dodder,  Cuscuta  europcea.  Part  of  a  Plant  paras;tic  on  a 
Branch  of  Willow,  with  germinating  Seedlings  on  the  right  and 
Section  of  Host  and  Parasite  on  the  left.  (From  Strasburger.) 

b,  vestigial  leaves;  SI,  flowers;  Cus,  stem  of  parasite  in  section ;  H,  haustoria  of  parasite 
in  section ;  W,  stem  of  host  in  section,  with  vascular  bundles  (v,  c) ;  t,  seedlings. 

one  of  the  most  frequent  results,  or  at  any  rate  concomitants,  of 
parasitism,  and  one  which  is  well  exemplified  in  the  case  of 
Sacculina,  is  hermaphroditism,  which  affords  many  more  chances 
for  the  fertilization  of  the  eggs  than  the  unisexual  condition. 

We  meet  with  precisely  analogous  phenomena  in  the  case  of 
many  parasitic  plants.     In  the  dodder  (Fig.  184)  the  leaves  and 


INSUFFICIENCY   OF   NATURAL   SELECTION      403 

roots  have  disappeared  almost  completely  and  no  chlorophyll  is  pro- 
duced, but  special  nutritive  organs,  the  sucker-like  haustoria,  are 
developed  on  the  slender,  twining  stems,  and  serve  to  extract  the 
necessary  food  from  the  host  plant.  The  flowers,  however,  upon 
which  the  perpetuation  of  the  race  depends,  still  remain  in  a 
well  developed  condition. 

Both  Sacculina  and  the  dodder  have  lost  the  power  of  inde- 
pendent existence,  and  if,  for  any  reason,  they  were  to  find 
themselves  suddenly  confined  to  an  environment  where  there  were 
no  suitable  hosts,  their  races  would  inevitably  become  extinct. 
(Naturejwould  treat  them  just  as  she  is  treating  the  wingless 
birds,  and  make  them  pay  the  penalty  for  the  degeneration  which 
they  have  undergone. 

*  It  has  sometimes  been  pointed  out  as  an  objection  to  the  theory 
of  natural  selection  that  it  cannot  account  for  the  first  origin  of 
favourable  variations.  The  theory  takes  variations  for  granted 
and  assumes  that  some  will  be  favourable  and  some  not,  that  the 
former  will  be  fostered  and  accumulated  from  generation  to 
generation  and  the  latter  ruthlessly  eliminated.  It  is  further 
alleged  that  variations  are  usually  so  slight  at  their  first  appear- 
ance that  they  can  have  no  selective  value,  and  that  something 
is  wanted  to  account  for  the  increase  of  such  variations  along 
apparently  definite  lines  of  utility.  The  theory  also  takes  the 
inheritance  of  variations  for  granted,  and  many  people,  as  we 
have  seen,  consider  nowadays  that  this  is  not  altogether  a 
justifiable  proceeding,  that  while  some  variations  undoubtedly 
are  inherited,  others,  and  amongst  them  many  which  would  be 
likely  to  be  of  the  greatest  value  to  the  organism,  are  not. 

We  have,  then,  to  go  much  deeper  than  the  idea  of  natural 
selection  before  we  can  reach  a  satisfactory  working  hypothesis 
as  to  the  manner  in  which  organic  evolution  has  taken  place. 
The  problems  of  variation  and  heredity  have  already  been  dealt 
with  in  earlier  chapters,  and  it  will  be  unnecessary  to  discuss  the 
matter  now  at  great  length,  but  there  are  certain  points  which 
we  must  recapitulate  in  this  connection. 

We  have  seen  that  somatogenic  or  bodily  variations  in  the 

;  individual  are  undoubtedly  brought  about  by  the  direct  action  of 

I  the  environment  and  by  the  use  and  disuse  of  organs.     We  have 

1 also  seen  that  blastogenic  variations,  which  originate  in  the  germ 

plasm,  may  likewise   be   brought   about  by   the   action   of  the 

D  D  2 


404        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

environment  (as  in  the  case  of  the  potato  beetle  as  demonstrated 
by  Tower),  but  that  they  probably  also  arise  from  the  mingling 
of  different  streams  of  ancestral  tendencies  in  the  process  of 
amphimixis  or  conjugation  of  gametes,  and  possibly  in  yet  other 
ways  with  which  we  are  not  acquainted. 

Of  course,  natural  selection  can  offty  influence  a  species 
through  variations  which  are  capable  of  being  inherited,  and  it 
is,  as  everyone  knows,  urged  by  many  modern  writers  that 
somatogenic  variations,  due  either  to  the  direct  action  of  the 
environment  or  to  the  use  and  disuse  of  parts,  cannot  be 
inherited  and  therefore  have  no  significance  in  evolution,  and 
that  natural  selection  must  content  herself  with  such  fortuitous 
.and  non-adaptive  variations  a*,  may  happen  to  arise  in  the  germ 
Iplasm.  This  indeed  se'ems  an  extreme-  view,  and  it  is  just 
[here  that  the  split  between  the  extreme  selectionists,  who  have 
gone  far  beyond  Charles  iWwinin  this  matter,  and  the  followers 
of  Lamarck  arises.  The  curious  thing  about  the  controversy  is 
that  there  is  no  inherent  incompatibility  between  the  views  of 
the  two  schools.  Tha^theory  of  the  inheritance,  to  a  limited 
extent,  of  acquired  characters,  indeed,  appears  to  be  just  what  is 
necessary  to  supply  the  demliencies  of  that  of  natural  selection. 

To  say  that  acquired  characters  cannot  be  inherited  because 
we  cannot  see  them  being  inherited  in  our  own  brief  lifetimes1  is 
like  saying  that  a  glacier  does  not  move  because  we  do  not  see  it 
or  feel  it  moving  as  we  walk  over  it.  I  have  endeavoured  to 
show  in  an  earlier  chapter  that  it  is  not  difficult  to  imagine  a 
mechanism  by  which  somatogenic  characters  may  gradually  be 
converted  into  blastogenic  ones,  and  if  this  is  in  any  way 
possible  there  is  no  reason  why  we  should  deny  the  possibility 
of  their  inheritance.  No  one,  however,  would  be  rash  enough 
to  suppose  that  all  that  an  animal  or  plant  acquires  in  its 
individual  lifetime  is  transmitted  to  its  heirs.  Nature  imposes 
a  heavy  death  duty  and  takes  away  by  far  the  greater  part  of  the 
capital  which  has  been  accumulated  by  each  individual.  We 
may  suppose,  however,  that  a  fraction  remains,  however 
unrecognizable  by  our  limited  powers,  and  that  these 
fractions,  accumulating  under  the  same  influences  throughout 
thousands  of  generations,  ultimately  confer  upon  the  organism 
as  a  birthright  that  adaptation  which  is  essential  to  its  existence. 

1  As  a  matter  of  fact,  it  appears  from  recent  experiments  that  in  some  cases  we 
can  see  them  being  inherited  (vide  p.  182). 


CO-OPERATION   OF   FACTORS  405 

Even  the  individual  can  do  much  in  its  own  lifetime  to  adapt  \ 
itself  to   its   environment,   and   when   the   residua   of    all   the  » 
individual  adaptations  are  summed  up  by  inheritance  the  result 
is  such  that  we  may  well  wonder  how  it  can  have  been  produced. 
TVivrmgfro.yifr  foo  wVHfl  process,  of  course,  natural  selection  must 
help  by  constp^y  wQ^'"g  ™it  inferiority,   but  it  ig^jiT^hg/hly 
the  direct  influence  of  frhe  environment,  including  the  use  and 
disuse  of  organs  in  response  to  that  influence,  that  is  in  most 
cases  the  determining  factor  in  bringing  about  adaptation. 

It  may  well  be,  however,  that  there  are  also  cases  in  which 
natural  selection  alone,  acting  through  the  occurrence  of  purely 
fortuitous  variations,  has,  in  the  struggle  for  existence,  ^been 
sufficient  to  produce  marvellous  adaptations.  This  may  have 
been  the  case  with  protective  resemblance  and  mimicry  in  form 
and  colour,  and  with  the  adaptation  of  flowers  for  fertilization  by 
insects,  in  all  of  which  it  is  difficult  to  see  how  the  direct  action 
of  the  environment  or  the  use  and  disuse  of  organs  could  bring 
about  adaptive  modifications.  But  the  factjihat  na.tnrfl.1  ap.lAp.f.TQn 
alone__appears  to  ha.vp.  hc^n  sufficient  in  some  cases  must  not 
preventus  from  admitting  the  action  of  other  factors  in  other 
cases^ The  fact  that  some  carriages  are  pulled  by  motors  affords 
no  justification  for  asserting  that  other  carriages  may  not  be 
pulled  by  horses,  or  that  the  same  carriage  may  not  at  one 
time  be  pulled  by  a  motor  and  at  another  by  a  horse,  or  even 
by  both  together.  Many  factors  must  have  co-operated  to 
bring  about  such  a  marvellously  complex  result  as  the  present 
condition  of  the  organic  world,  and  no  sufficient  reason  has 
yet  been  shown  for  denying  ourselves  the  assistance  of 
"  Lamarckian  "  factors  in  our  endeavours  to  discover  the  processes 
through  which  this  result  has  come  about.1 
*  We  may  now  turn  our  attention  to  a  group  of  cases  which 
certainly  appear  to  support  the  view  that  the  factors  at  work  in 
determining  any  particular  line  of  evolution  are  more  complex 
than  might  at  first  sight  be  supposed.  It  is  a  fact  well  known 
to  palaeontologists  that  many  widely  separated  groups  of  the 
animal  kingdom  have,  during  the  course  of  their  evolution,  and 
especially  towards  the  end  of  that  course,  shown  a  strongly 
marked  tendency  to  enormous  increase  in  size.  We  see  this 
in  the  extinct  eurypterids  (Fig.  137),  giants  amongst  the 

1  "  To  insist  on  ascribing  complex  results  to  single  causes  is  the  well-known  vice 
of  narrow  and  untrained  minds  "  (Morley's  li  Life  of  Gladstone,"  Vol.  II.,  p.  68). 


406        OUTLINES   OF   EVOLUTIONARY    BIOLOGY 

arthropods;  in  the  huge  labyrinthodont  amphibians;  in  many 
groups  of  reptiles  of  the  Secondary  period,  some  of  which 
attained  a  length  of  80  feet  or  more,  and  amongst  mammals  in 
the  extinct  Tinoceras  (Fig.  150)  and  the  still  surviving  elephants 
and  whales.  Comparative  anatomists  are  familiar  with  similar 
phenomena  exhibited  by  individual  organs,  such  as  the  extra- 
ordinary development  of  horns  and  spines  in  many  of  the 
extinct  reptiles  referred  to  (Fig.  145),  the  immense  tusks  of 


FIG.   185. — Head    of    Bablrusa  alfurus.      (From    Flower  and  Lyddeker's 
"  Mammals  Living  and  Extinct.") 

the  babirusa  (Fig.  185),  and  the  gigantic  and  grotesque  beak 
and  "  helmet "  of  the  hornbill  (Fig.  186). 

The  exuberant  development  of  some  organs  of  this  kind  may 
possibly  be  attributed  to  the  action  of  sexual  selection,  and  indeed 
our  daily  experience  of  our  own  species  seems  to  warrant  us  in 
believing  that  there  is  no  limit  to  the  grotesque  results  which 
may  ensue  from  the  unrestricted  exercise  of  the  aesthetic  faculties 
of  either  sex,  but  it  seems  hardly  reasonable  to  attempt  to 
explain  all  such  bizarre  and  monstrous  productions  in  this 
manner. 

In  all  the  cases  cited,  and  in  many  others  which  could  be 
adduced,  either  the  entire  body  or  some  particular  organ  appears 
to  have  acquired  some  sort  of  momentum,  by  virtue  of  which  it 
continues  to  grow  far  beyond  the  original  limits  of  utility,  although 
perhaps  in  some  cases  a  new  use  may  be  found  which  will  assist 
the  species  in  maintaining  itself  in  the  struggle  for  existence. 


EXTINCTION   OF   GIANT  EACES 


407 


An  enormous  increase  of  mere  bodily  size,  however,  seems  in  the 
long  run  to  be  always  fatal  to  the  race,  whose  place  will  be  taken 
by  smaller  and  presumably  more  active  forms.  The  gigantic 
amphibians  are  all  extinct,  so  are  the  really  gigantic  reptiles, 
and  of  the  gigantic  mammals  only  a  couple  of  species  of  elephants 


FIG.  186. — A  Hornl>ill,  Buceros  rJiinoceros,  from  North- West  Borneo.    (Drawo 
from  a  specimen  in  the  British  Museum,  Natural  History.) 

and  a  few  whales  survive,  all  of  which  are  being  rapidly  exter- 
minated in  competition  with  man. 

There  is  perhaps  some  justification  in  recent  developments  of 
physiological  science  for  the  belief  that  a  race  of  animals  may 
acquire  a  momentum  of  the  kind  referred  to  ;  that  some  brake 
is  normally  applied  to  the  growth  of  organisms  and  organs  and 
that  sometimes  this  brake  is  removed,  leaving  the  organism  to 
rush  onwards  to  destruction  like  a  car  running  away  down  hill. 

Many  modern  physiologists  hold  the  view  that  the  growth  of 


408        OUTLINES   OF   EVOLUTIONAKY  BIOLOGY 

the  different  parts  of  the  animal  body  is  affected  by  internal 
secretions,  or  hormones,  the  products  of  various  glands.  Thus 
we  know  that  hypertrophy  of  the  pituitary  body  in  man  may 
lead  to  acromegaly,  one  of  the  symptoms  of  which  is  great 
enlargement  of  certain  parts.  The  most  dreadful  of  all  the 
diseases  to  which  human  beings  are  subject,  cancer,  is  essentially 
due  to  an  unrestrained  multiplication  of  cells,  and  consequent 
abnormal  growth  of  tissue,  which  may  very  possibly  be  corre- 
lated with  the  extent  to  which  some  specific  internal  secretion 
is  produced  in  the  body.  It  also  seems  not  unreasonable  to 
suppose  that  the  growth  of  the  body  may  be  normally  inhibited 
or  checked  by  specific  secretions,  and  that  in  the  absence  of  these 
it  may  continue  far  beyond  the  ordinary  limits. 

It  is  difficult  to  see  any  good  reason  why  we  should  not  apply 
this  principle  to  the  race  as  well  as  to  the  individual,  and, 
paradoxical  as  it  may  appear,  it  even  seems  possible  to  explain 
both  the  growth  of  the  organism  as  a  whole  and  that  of  its 
various  organs,  beyond  the  limits  of  utility,  as  an  indirect  result 
of  natural  selection. 

When  a  useful  organ,  such  as  the  tusk  of  a  wild  boar,  is  first 
beginning  to  develop,  or  to  take  on  some  new  function  for  the 
execution  of  which  an  increase  in  size  will  be  advantageous, 
natural  selection  will  favour  those  individuals  in  which  it  grows 
most  rapidly  and  attains  the  largest  size  in  the  individual  lifetime. 
If  growth  is  normally  checked  and  controlled  by  some  specific 
secretion,  or  hormone,  natural  selection  will  favour  those 
individuals  in  which  the  glands  which  produce  this  secretion  are 
least  developed,  or  at  any  rate  least  active.  The  process  being 
repeated  from  generation  to  generation  these  glands  (whatever 
may  be  their  nature,  and  we  use  the  term  "  gland  "for  any  cell 
or  group  of  cells  which  produces  a  specific  secretion,  whether 
recognizable  as  a  distinct  organ  or  not)  may  ultimately  be 
eliminated,  or  at  any  rate  cease  altogether  to  produce  the  par- 
ticular hormone  in  question.  Moreover,  this  elimination  may 
take  place  long  before  the  organ  whose  growth  is  being  favoured 
by  natural  selection  has  reached  the  optimum  size.  When  it 
has  reached  this  optimum  it  is  certainly  desirable  that  it  should 
grow  no  larger,  but  is  there  now  any  means  by  which  further 
growth  can  be  checked  ?  The  inhibiting  hormone  is  no  longer 
produced ;  the  brake  has  been  removed,  and  further  growth  may 
be  supposed  to  take  place  irrespective  of  utility,  until,  when 


EXCESSIVE  GKOWTH  409 

the  size  of  the  organ  gets  too  great  to  be  any  longer  compatible 
with  the  well-being  of  the  race,  natural  selection  again  steps  in 
and  eliminates  the  race.  The  same  argument  of  course  applies 
to  the  size  of  the  body  as  a  whole  as  well  as  to  that  of  its  con- 
stituent parts. 

It  may  be  thought  that  many  of  the  bizarre  and  almost 
monstrous  characters  under  discussion,  such,  for  example,  as 
some  of  the  excrescences  of  the  dermal  armature  in  extinct 
reptiles  (Fig.  145),  can  never  have  had  any  value  as  adaptations, 
and  that  therefore  natural  selection  could  never  have  encouraged 
them  to  increase  so  much  in  size  as  to  get  beyond  her  control. 
Here,  however,  the  principle  of  correlation  comes  in.  \  Just  as 
many  totally  different  organs  are  affected  by  disease  of  the 
pituitary  body,  so  the  removal  of  the  gland  which  controlled  the 
development  of  some  undoubtedly  useful  organ,  such  as  a  frontal 
horn,  might  at  the  same  time  permit  the  growth  of  all  sorts  of 
excrescences  which  have  no  adaptive  significance.  ^ 

Thus  it  appears  not  impossible  that,  the  normal  checks  to 
growth  being  removed  along  certain  lines  by  the  action  of  natural 
selection,  a  definite  direction  might  be  given  to  the  course  of 
evolution,  which  the  organism  would  continue  to  follow  irrespec- 
tive both  of  natural  selection  and  of  the  principle  of  use  and 
disuse.1 

In  the  present  state  of  our  knowledge,  however,  the  above 
suggestions  can  only  be  regarded  as  tentative.  They  are  doubtless 
open  to  much  criticism,  and  it  is  unfortunately  impossible  to 
subject  them  to  the  crucial  test  of  experiment. 

i  I  have  discussed  this  question  at  somewhat  greater  length  in  a  paper  read 
before  the  British  Association  for  the  Advancement  of  Science  (vide  Keport  of  the 
Portsmouth  Meeting.  1911). 


CHAPTEK  XXVII 

Artificial  selection — Continuous  and  single  selection — The  mutation  theory 
of  the  origin  of  species  —  Mutual  adaptation  —  Unit  characters  — 
Isolation  —  Physiological  selection  —  Non-adaptive  characters  —  The 
evolution  of  man. 

IT  has  long  been  recognized  that  much  light  may  be  thrown 
upon  the  problem  of  the  origin  of  species  by  the  careful 
study  of  the  methods  which  mankind  has  adopted  for  the 
improvement  of  the  various  races  of  cultivated  plants  and 
domesticated  animals.  Many  such  races  have  been  so  greatly 
modified  that,  did  they  occur  in  what  is  commonly  called  a  state 
of  nature,  we  should  be  obliged  to  regard  them  as  distinct  species. 
The  history  of  some  of  these  is  lost  in  antiquity  and  we  have  no 
positive  knowledge  of  the  methods  by  which  the  improvement  of 
wild  species  was  first  effected.  We  may  assume,  however,  with 
some  degree  of  confidence,  that  the  earliest  breeders  and 
cultivators  would  select  for  cultivation  and  propagation  those 
individuals  which  offered  them  the  most  valuable  qualities,  and 
that  they  would  reject  such  as  exhibited  marked  signs  of 
inferiority.  This  process,  repeated  from  generation  to  generation 
through  thousands  of  years,  and  aided  in  each  generation  by  the 
direct^  effects  of  cultivation,  could  not  fail  to  bring  about  con- 
spicuous results.  For  an  account  of  what  has  been  effected 
in  this  manner  the  student  should  consult  Charles  Darwin's 
classical  work  on  the  "  Variation  of  Animals  and  Plants  under 
Domestication." 

The  almost  unconscious  efforts  of  our  ancestors  have  given 
place  in  modern  times  to  deliberate  and  systematic  attempts  to 
discover  the  principles  upon  which  the  improvement  of  cultivated 
races,  both  of  plants  and  animals,  should  be  based. 

Perhaps  no  species  of  plants  have  been  more  improved  by 
man  than  the  various  cereals  upon  which  he  relies  so  largely  for 
his  food  supply.  Professor  de  Vries,  in  his  interesting  book  on 
"  Plant-Breeding,"  1  describes  how  such  improvement  has  been 

JKegan  Paul,  Trench,  Trubner  &  Co.,  London,  1907. 


CONTINUOUS    SELECTION  411 

effected  in  recent  times.  In  the  first  place  much  care  and 
thought  have  been  devoted  to  carrying  out  experiments  in 
accordance  with  the  principle  of  continuous  selection : — 

"  The  general  custom  [in  Germany]  was  to  start  such  experi- 
ments from  the  best  local  or  improved  varieties  by  an  initial  choice 
of  a  certain  number  of  typical  heads.  Such  a  group  of  selected 
plants  was  called  the  elite,  and  this  elite  had  to  be  ameliorated 
according  to  the  prevailing  demands  or  even  simply  in  accordance 
with  some  ideal  model.  Year  after  year,  the  best  ears  of  the 
elite  group  were  chosen  for  the  continuance  of  the  strain  or 
family,  and  slowly,  but  gradually,  its  qualities  were  seen  to 
improve  in  the  desired  direction.  After  some  years,  such  a 
family  might  become  decidedly  better  than  the  variety  from 
which  it  had  been  derived.  Then  its  yearly  harvest  would  be 
divided  into  two  parts,  after  having  been  sufficiently  purified  by 
the  rejection  of  accidental  ears  of  minor  worth.  The  best  ears 
were  carefully  sought  out  and  laid  aside  for  the  continuance  of 
the  elite  strain,  but  the  remainder  were  sown  on  a  distant  field 
in  order  to  be  multiplied  as  fast  as  possible.  By  this  means,  after 
a  multiplication  during  two  or  three  generations,  its  product 
could  be  used  as  seed  grain  for  the  farm  or  sold  to  others  for 
the  same  purpose.  Each  year  the  elite  would,  of  course,  give  a 
new  and  better  harvest  which  could  be  multiplied  and  sold  in  the 
same  manner." 1 

By  this  method  improvement  may  undoubtedly  be  effected, 
but  the  selection  has  to  be  constantly  repeated,  otherwise  the 
improved  strain  rapidly  deteriorates  again.  Indeed  it  may  be 
questioned  whether  it  is  possible  in  this  way  to  effect  any  per- 
manent improvement,  at  any  rate  in  the  case  of  cereals.  One 
reason  for  this  appears  to  be  that  we  are  dealing  all  the  time, 
not  with  a  single  pure  race,  but  with  a  mixture  of  distinct  races. 
We  must  also  remember  that  many  of  the  characters  which  it  is 
desired  to  perpetuate  and  increase  may  be  the  direct  result  of  the 
cultural  methods  employed,  and,  as  we  have  already  seen,  we 
cannot  expect  such  causes  to  produce  visibly  heritable  effects  in 
the  course  of  a  few  generations,  whatever  they  might  do  in  the 
long  run. 

We  have  had  occasion  to  point  out  in  an  earlier  chapter  that, 
according  to  Professor  de  Vries,  new  species  arise,  in  a  state  of 
nature,  not  by  the  accumulation  in  particular  directions  of  small, 
fluctuating  variations,  but  by  the  sudden  appearance  of  those 

»  De  Vries,  op.  cit.,  p.  58. 


412        OUTLINES   OF   EVOLUTIONARY  BIOLOGY 

more  conspicuous  variations  known  as  mutations.  De  Tries 
points  out  that  many  of  the  so-called  Linnean  species, 
such  as  Draba  verna,  are  in  reality  made  up  of  a  large 
number  of  "elementary  species"  which  have  arisen  in  this 
manner,  and  certain  results  which  have  been  obtained  in  experi- 
ments upon  the  improvement  of  cereals  appear  at  first  sight  to 
afford  considerable  support  to  these  views. 

It  has  long  been  known  that  an  ordinary  field  of  wheat  con- 
tains a  larger  or  smaller  number  of  "  types,"  "  mutations,"  or 
"  elementary  species,"  which  can  be  recognized  by  the  experienced 
eye,  and  it  has  been  shown  that  if  a  single  plant  of  one  of  these 
types  be  isolated  it  will  produce  offspring  like  itself  and  continue 
to  breed  true  for  an  indefinite  number  of  generations.  Of  course 
it  is  necessary  that  there  should  be  no  crossing  with  other 
types,  but  this  is  easily  avoided,  for,  although  accidental 
crosses  may  occur,  the  cereals,  with  the  exception  of  rye,  are 
usually  self-fertilizing.  Upon  this  knowledge  is  founded  the 
method  of  single  as  opposed  to  continuous  selection,  a  single 
selection  of  a  suitable  type  being  enou-gh  to  establish  the  desired 
strain. 

One  of  the  first  to  make  use  of  the  method  of  single  selections 
was  Patrick  Shirreff : — 

"  His  first  discovery  was  made  in  the  year  1819.  He  observed 
a  plant  of  wheat  which  surpassed  its  neighbors  by  its  high 
degree  of  branching.  It  yielded  63  ears  with  about  2500 
kernels.  He  saved  the  seeds,  sowed  them  on  a  separate  field 
and  at  considerable  distances  apart  so  as  to  induce  in  all  the 
plants  the  same  rich  branching.  He  contrived  to  multiply  it  so 
rapidly  that  it  took  only  two  generations  to  get  seed  enough  to 
bring  it  advantageously  into  the  trade.  He  gave  it  the  name  of 
Mungoswell's  wheat,  and  it  soon  became  one  of  the  most  profit- 
able varieties  of  Scotland.  It  has  found  its  way  into  England 
and  into  France,  where  it  is  still  considered  one  of  the  best  sorts 
of  wheat.'' 1 

The  same  method  has  been  subjected  to  severe  tests  and 
placed  upon  a  thoroughly  scientific  footing  at  the  Swedish 
Agricultural  Station  of  Svalof. 

We  have  seen,  in  Chapter  XIV,  that  hybridization  may 
occasionally  give  rise  to  permanent  races  or  strains  exhibiting 
new  combinations  of  characters,  and  that  this  takes  place  in 

1  De  Vries,   op.  cit.,  pp.  34,  35. 


HYBRIDIZATION  AND   MUTATION  413 

accordance  with  Mendelian  principles.  It  cannot  be  doubted  that 
hybridization  occurs  occasionally  in  cultivated  cereals,  and 
Professor  de  Vries  is  of  opinion  that  the  occurrence  of  the 
different  types  or  mutations1  is  often  the  result  of  hybridization 
at  various  periods  in  the  history  of  the  race  : — 

"Experience,  however,  shows  that  in  ordinary  fields  almost 
all  possible  combinations  may  be  met  with,  and  it  is  to  be  pre- 
sumed that  at  least  the  greater  number  of  them  are  due  to 
crosses  in  previous  and,  perhaps,  in  long-forgotten  years."  2 

Some  of  these  combinations,  as  might  be  expected,  are  not 
stable  but  split  up  into  a  number  of  varieties  in  the  next 
generation,  but  also  : — 

"  We  may  conclude  that  some,  and  perhaps  many,  of  the  types 
which  may  be  selected  and  isolated  in  the  fields  and  which  prove 
to  be  constant  races  must  be  of  hybrid  origin."  2 

De  Vries  maintains  that  in  the  case  of  the  cereals  so  many  of 
these  "  types  "  now  lie  ready  to  our  hand  that  all  we  have  to  do 
is  to  pick  out  those  which  we  require  and  cultivate  them  in 
isolation  from  each  other  and  from  the  remainder.3  Professor 
Biffen  has  shown,  however,  as  we  have  already  pointed  out  in 
Chapter  XIV,  that  it  is  possible  by  intelligent  artificial  hybridi- 
zation to  produce  yet  other  stable  combinations  or  hybrids 
which  may  surpass  in  value  any  which  have  accidentally  arisen 
in  the  past. 

It  appears,  then,  that  many  at  any  rate  of  the  so-called 
mutations  or  types  amongst  cereals  are  due  to  hybridization. 
How  far  this  applies  to  mutations  in  general  it  is  quite  impossible 
to  decide.  That  it  is  not  always  so,  however,  appears  to  be  proved 
by  the  occurrence  of  such  mutations  or  sports  as  hexadactylism, 
which  are  known  to  be  inherited  and  which  cannot  have 
arisen  in  this  way. 

De  Vries  tells  us  in  another  work  that : — 

"  According  to  the  theory  of  mutation  species  have  not  arisen 
gradually  as  the  result  of  selection  operating  for  hundreds,  or 
thousands,  of  years  but  discontinuously  by  sudden,  however 
small,  changes.  In  contradistinction  to  fluctuating  variations 

1  De  Vries,  op.  cit.,  p.  322. 

2  Ibid.,  p.  80. 

3  1  bid.,  p.  50.     It  seems  strange,  considering  that  de  Vries  admits  that  many 
at  any  rate  of  the  "types"  have  probably  arisen  by  hybridization  in  the  first 
instance,  that  he  should  attribute  so  little  value  to  artificial  hybridization  as  a 
means  of  improvement. 


414        OUTLINES   OF   EVOLUTIONAEY  BIOLOGY 

which  are  merely  of  a  plus  or  minus  character  the  changes  which 
we  call  mutations  are  given  off  in  almost  every  manner  of  new 
direction.  They  only  appear  from  time  to  time,  their  periodicity 
being  probably  due  to  perfectly  definite  but  hitherto  undiscovered 
causes. 

"  The  theory  of  the  inheritance  of  acquired  characters  comes 
under  the  heading  of  fluctuations.  Acquired  characters  have 
nothing  to  do  with  the  origin  of  species.  Nor  can  the  theory  o| 
descent  be  applied  to  the  solution  of  social  problems." * 

There  is  here  no  suggestion  of  a  hybrid  origin  for  the 
mutations  in  question.  If,  however,  as  seems  probable,  a  large 
proportion  of  so-called  mutations  are  really  the  result  of  hybridi- 
zation, and  if,  as  we  showed  in  Chapter  XIV,  hybrids  tend  to  be 
automatically  eliminated  in  a  state  of  nature — though  of  course 
there  is  nothing  to  prevent  a  constant  hybrid  from  being  pre- 
served if  it  happens  to  possess  characters  peculiarly  favourable 
to  its  own  existence — it  does  not  seem  likely  that  such  mutations 
can  have  played  any  very  great  part  in  organic  evolution.  In 
any  case  there  is  no  need  to  suppose  that  the  theories  of  muta- 
tion and  natural  selection  are  mutually  exclusive,  for,  however 
new  characters  may  arise,  they  must  be  subject  to  the  action  of 
natural  selection  in  the  struggle  for  existence. 

VProfessor  de  Vries'  objection  to  small,  fluctuating  variations  as 
the  material  upon  which  natural  selection  operates  in  the 
modification  of  species  appears  to  be  based  upon  the  view  that 
such  characters  are  acquired  in  the  lifetime  of  the  individual 
and  cannot  be  inherited,  v.  If,  however,  we  admit  that  a  soma- 
togenic  character  may,  in  the  course  of  many  generations 
and  under  the  continued  influence  of  the  same  conditions  which 
originally  called  it  forth,  become  converted  into  a  blastogenic 
character,  this  difficulty  entirely  disappears. 

It  is  extremely  hard  to  believe  that  mutations,  which,  apart 
from  the  occurrence  of  hybridization,  seem  to  occur  very  rarely 
and  at  long  intervals,  can  have  afforded  sufficient  opportunity 
for  the  production  of  all  the  marvellous  adaptations  which  exist 
in  nature.  Take,  for  example,  themutual  adaptations  which  we 
see  between  the  length  of  the  nectary  in  certain  flowers  and  the 
length  of  the  proboscis  in  the  insects  which  fertilize  them.  We 
cannot  suppose  that  either  the  elongated  nectary  or  the  elongated 
proboscis  arose  by  sudden  mutations,  for  unless  these  mutations 

i  "  The  Mutation  Theory."     English  Trans.,  Vol.  I.,  p.  213. 


MUTATION   AND   ADAPTATION  415 

took  place  simultaneously  in  flower  and  insect,  and  in  the  same 
locality,  and  in  a  sufficient  number  of  each,  a  supposition  which, 
to  say  the  least  of  it,  is  wildly  improbable,  the  delicate  correla- 
tion between  the  two  would  be  thrown  out  of  gear  and  the 
individuals  exhibiting  the  mutations  would  be  eliminated  by 
natural  selection  because  they  were  no  longer  sufficiently  well 
adapted  to  the  very  special  conditions  of  their  environment. 

We  can  only  believe  that  the  increase  in  length  of  nectary  and 
proboscis  took  place  so  slowly  that  their  reciprocal  adaptation 
was  never  upset.  A  slight  increase  in  the  length  of  the.  nectary 
obliged  the  insect  to  poke  further  into  the  flower  for  the  honey 
and  thus  increased  the  chances  of  fertilization.  A  slight  increase 
in  the  length  of  the  proboscis  enabled  the  insect  to  get  more 
honey  and  thus  gave  it  a  better  chance  of  existence.  After 
perhaps  many  thousands  of  generations,  under  the  influence  of 
natural  selection,  combined  in  the  case  of  the  insect  with  the 
effects  of  use  and  disuse,  the  present  enormous  lengths  of 
proboscis  and  nectary  have  been  attained,  and  no  one  doubts  the 
fact  that  they  have  become  blastogenic  characters. 

The  same  argument  applies  to  all  accurate  adaptations  to 
special  conditions  of  the  environment.  How  can  we  explain  the 
facts  of  protective  resemblance  and  mimicry  except  as  due  to  the 
accumulation  under  the  influence  of  natural  selection  of  what 
Charles  Darwin  called  slow  successive  variations  ?  How,  again, 
can  the  theory  of  mutation  be  applied  to  such  cases  as  that  of 
the  flightless  birds  on  oceanic  islands?  Who  can  doubt  that 
the  reduction  of  the  wings  and  the  loss  of  the  power  of  flight  has 
been  brought  about  slowly  and  gradually  as  a  result  of  disuse  ? 
and  at  the  same  time  who  would  venture  to  argue  that  the 
flightless  birds  are  not  specifically  distinct  from  their  actively 
flying  ancestors  ?  If  it  be  urged  that  mutations  may  be  so  small 
as  to  be  almost  imperceptible,  then  we  must  ask  how  do  they 
differ  from  fluctuating  variations  ?  and  if  we  are  told  that  they 
occur  very  rarely  and  do  not  fluctuate,  that  very  answer  is 
sufficient  to  show  that  they  can  hardly  have  given  rise  to  adaptive 
modifications. 

De  Yries'  theory  of  the  origin  of  species  by  mutation  is 
supposed  to  harmonize  with  the  Mendelian  principle  of  unit 
characters,  but  we  have  to  ask  ourselves,  how  do  new  unit 
characters  arise  in  the  first  instance?  It  seems  at  least  as 
probable  that  they  arise  by  the  gradual  accumulation  of  slight 


416        OUTLINES   OF  EYOLUTIONAEY  BIOLOGY 

fluctuating  variations  under  the  control  of  natural  selection  as 
that  they  originate  in  any  other  way  that  can  be  suggested  in 
the  present  state  of  our  knowledge  ;  and  even  if  these  variations 
are  at  first  purely  somatogenic,  we  may  suppose  that  in  the 
course  of  many  generations  they  gradually  exert  a  cumulative 
influence  upon  the  germ  plasm  until  the  latter,  so^to  speak, 
topples  over  into  some  new  position  of  equilibrium  and  a  new 
unit  character  arises.  We  cannot,  however,  now  add  to  what  we 
have  already  said  on  this  subject  when  dealing  with  the  theory 
of  heredity. 


One  of  the  most  important  factors  in  bringing  about  divergent 
evolution  is  undoubtedly  isolation.  However  a  new  character 
may  have  arisen  it  is  liable  to  be  swamped  by  the  crossing  of 
the  individuals  which  possess  it  with  others  which  do  not  possess 
it  unless  by  some  means  or  other  the  two  groups  are  prevented 
from  interbreeding.  We  cannot  fail  to  see  the  importance  of  this 
principle  when  we  study  the  fauna  and  flora  of  remote  islands, 
and  their  relationships  to  those  of  the  nearest  continental  areas 
or  of  other  islands. 

Take,  for  example,  the  case  of  the  Chatham  Islands,  which,  as  we 
have  already  seen,  lie  some  400  miles  to  the  east  of  New  Zealand. 
There  can  be  very  little  doubt  that  these  islands  were  formerly 
connected  with  the  New  Zealand  mainland,  and  this  connection 
probably  continued  into  Pleistocene  times,  when  a  great 
depression  took  place  which  caused  the  two  to  be  separated  by 
a  wide  tract  of  ocean.  All  who  have  studied  the  question  are 
agreed  that  the  fauna  and  flora  of  the  Chatham  Islands  are 
simply  isolated  detachments  of  those  of  New  Zealand.  Many 
species,  especially  of  the  plants,  are  identical  with  New  Zealand 
species,  but  many  others,  though  closely  related  to  those  of  New 
Zealand,  are  considered  by  systematists  to  be  specifically  distinct, 
and  they  occur  nowhere  else  in  the  world. 

We  have  here  an  excellent  illustration  of  the  effects  of 
geographical  isolation,  which  we  shall  be  able  to  appreciate 
better,  perhaps,  if  we  confine  our  attention  to  a  few  typical  cases. 
The  common  New  Zealand  wood  pigeon,  Carpophaga  (Hemiphaga) 
nov<s-zealandi<z,  is  represented  on  the  Chathams  by  a  species 
known  as  Carpophaga  (Hemiphaga)  chathamensis,  differing  but 
slightly  from  its  new  genus  congener,  and  the  New  Zealand 


GEOGRAPHICAL   ISOLATION  417 

lizard,  Lygosoma  moco,  is  represented  on  Pitt  Island  (one  of  the 
Chathams)  by  a  very  similar  form  described  by  Mr.  Boulenger 
under  the  name  Lygosoma  dendyi.  The  remarkable  New  Zealand 
lance-woods  (Pseudopanax  crassi/olium  and  P.  ferox)  are  repre- 
sented on  Chatham  Island  by  the  closely  related  Pseudopanax 
chathamicum,  and  the  Chatham  Island  ribbon-wood  also  differs 
slightly  from  the  common  New  Zealand  species  (Plagianthus 
betulinus). 

The  explanation  of  these  differences  is  that  the  two  portions 
into  which  each  of  the  species  mentioned  became  divided  when 
the  Chatham  Islands  were  separated  from  New  Zealand  have 
diverged  from  one  another  and  followed  somewhat  different  lines 
of  evolution.  Owing,  perhaps,  to  slightly  different  conditions  of 
the  environment,  or  to  other  causes  which  it  is  impossible  to 
specify,  one  or  both  has  become  modified  to  a  greater  or  less 
extent  in  its  own  particular  direction  and,  owing  to  the 
geographical  isolation,  there  has  been  no  interbreeding  between 
the  two  sections  to  keep  them  both  in  the  same  average  condition. 

The  principle  of  isolation  fully  explains  why  the  fauna  and  flora 
of  oceanic  islands  in  general  are  made  up  almost  entirely  of 
peculiar  species  found  nowhere  else  in  the  world.  The  ancestors 
of  these  species  were  originally  derived  from  some  very  distant, 
probably  continental  area,  and  their  descendants  have  had  few 
if  any  opportunities  of  interbreeding  with  the  parent  species, 
from  which  they  have  gradually  diverged  further  and  further 
under  their  new  conditions  of  life. 

Certain  writers,  such  as  Mr.  Gulick  and  Dr.  Romanes,  have 
maintained  that  the  mere  separation  of  a  species  into  two  or 
more  sections  which  are  prevented  from  interbreeding  would 
suffice  to  bring  about  divergent  evolution,  irrespective  of  whether 
or  not  the  separate  sections  were  exposed  to  different  environ- 
mental conditions.  It  would  probably  be  impossible  to  divide 
a  species  into  two  sections  whose  average  qualities  are  identical, 
and: — 

"  No  matter  how  infinitesimally  small  the  difference  may  be 
between  the  average  qualities  of  an  isolated  section  of  a  species 
compared  with  the  average  qualities  of  the  restof  that  species,  if 
the  isolation  continues  sufficiently  long,  differentiation  of  specific 
type  is  necessarily  bound  to  ensue." l 

1  Komanes,  "  Darwin  and  after  Darwin,"  Vol.  Ill,  "  Isolation  and  Physiological 
Selection,"  p.  13. 

B.  E   E 


418        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

If  isolation  is  necessary  for  the  establishment  of  new  species 
by  divergent  evolution,  why,  it  may  be  asked,  do  we  find  closely 
related  species,  which  we  must  suppose  to  be  descended  from 
common  ancestors  at  no  very  distant  period,  actually  inhabiting 
the  same  areas  at  the  present  time  ?  The  answer  is  that  there 
are  other  means  by  which  groups  of  organisms  can  be  prevented 
from  interbreeding  besides  geographical  isolation. 

It  used  to  be  supposed  that  one  of  the  best  tests  of  the  specific 
distinctness  of  any  two  forms  was  their  incapacity  for  breeding 
together  and  producing  fertile  offspring.1  The  mule,  it  is  true,' 
is  the  offspring  of  parents  belonging  to  two  distinct  species,  the 
ass  and  the  horse,  but  the  mule  is  almost  always  sterile,  and  most 
well  characterized  species2  are  incapable  of  breeding  together  at 
all.  This  fact  has  been  regarded  by  many  people  as  a  most  serious 
difficulty  in  the  way  of  comparing  the  origin  of  species  by  natural 
selection  with  the  results  produced  amongst  domesticated  plants 
and  animals  by  artificial  selection,  for  the  products  of  artificial 
selection,  however  much  they  may  differ  from  one  another,  if 
they  have  been  derived  from  the  same  parent  species  will 
remain  capable  of  breeding  together  with  perfect  fertility. 

The  solution  of  this  difficulty  is  to  be  found  in  the  theory  of 
"  Physiological  Selection,"  which  we  owe  to  Mr.  Gulick,  Dr. 
Romanes  and  others.  These  writers  point  out  that  amongst 
the  endless  variations  to  which  plants  and  animals,  are  subject 
will  be  variations  in  the  reproductive  system,  by  which  certain 
individuals  will  be  rendered  infertile  when  crossed  with  others 
of  the  same  species,  while  remaining  fertile  with  individuals 
which  have  varied  in  the  same  manner  as  themselves.  In  this  way 
a  species  may  be  as  effectively  divided  into  two  sections  as  by  any 
geographical  barrier,  and  under  these  circumstances  divergent 
evolution  may  be  expected  to  take  place.  According  to  this  view 
the  mutual  sterility  which,  to  a  greater  or  less  extent,  undoubtedly 
does  characterize  distinct  species  in  a  state  of  nature,  is  the 
cause  and  not  the  result  of  their  distinctness,  and  cannot  be 
regarded  as  a  reason  for  supposing  that  there  is  any  essential 
difference  between  the  processes  of  artificial  and  natural 
selection.  In  artificial  selection  it  is  merely  another  kind  of 

1  Although  Lamarck  pointed  out  a  century  ago  that  this  is  really  no  criterion  of 
specific  distinction.  The  idea  that  it  is  so  is  clearly  expressed  by  Buffon  ("  Histoifle 
Naturelle,"  IDHI.  VI,  1756,  p.  16). 

a  At  any  rate  amongst  the  higher  animals. 


NON-ADAPTIVE    CHAKACTERS  410 

isolation  that  has  been  employed  to  prevent  the  swamping 
effects  of  intercrossing ;  but  it  is  an  isolation  that  may  be 
broken  through  at  any  moment,  and  if  all  the  different  varieties 
of  some  domestic  plant  or  animal  were  turned  loose  to  struggle 
for  existence  and  interbreed  with  one  another  and  with  the 
parent  species  in  a  state  of  nature,  they  would  probably  in  most 
cases  very  soon  cease  to  have  any  separate  existence. 


The  theory  of  natural  selection,  combined  with  that_of  the 
gradual  inheritance  of  the  effects  of  use  a.r>d  dianaa  a.nri  nf  nf.hfir 
modifirajjonfl  brought  about  hy  thp.  Inng-finntinued  influenceof 
the  environment  affords  a  satisfactory  explanation  of  the  evglu- 
tioinyt~agaptive  characters.^  Many  if  not  all  organisms,  how- 
ever,  exhibit  characters  to  which  we  can  assign  no  adaptive  value, 
which  do  not  seem  to  be  of  any  particular  use  to  the  organism  in 
the  struggle  for  existence,  or  which  apparently  might,  so  far  as 
utility  is  concerned,  be  equally  well  replaced  by  any  one  of  a 
number  of  alternative  characters. 

Amongst  the  microscopic  Protozoa  species  are  frequently 
distinguished  from  one  another  by  minute  differences  in  the  form 
or  ornamentation  of  the  skeleton  (compare  Figs.  3  and  4). 
Different  species  of  the  genus  Lagena  (Fig.  4),1  amongst  the 
Foraminifera,  for  example,  exhibit  different  sculptured  patterns 
upon  their  flask-shaped  calcareous  shells.  Are  we  to  suppose 
that  it  is  of  any  consequence  to  the  gelatinous,  Amoeba-like 
inhabitant  of  the  flask  whether  its  shell  be  ornamented  in  one 
way  rather  than  in  another  ?  Does  one  pattern  help  a  uni- 
cellular foraminiferan  or  radiolarian  more  than  another  in  the 
struggle  for  existence  ?  The  same  argument  applies  to  the 
extremely  minute  siliceous  flesh -spicules  or  microscleres 
which  occur  scattered  without  order  through  the  ground  sub- 
stance of  many  sponges,  and  the  form  of  which  is  regarded  by 
those  who  have  studied  the  question  as  by  far  the  most  reliable 
guide,  not  only  in  the  recognition  of  species,  but  also  in  the 
grouping  of  these  species  in  genera  and  families.  Take,  for 
example,  the  wonderful  chelae  (Fig.  187),  characteristic  of  the 
family  Desmacidonidae.  Different  genera  and  species  are  dis- 
tinguished by  differences  in  the  size,  shape  and  number  of 

1  The  two  figures  below  the  centre  figure  and  two  in  the  bottom  right  hand 
corner  represent  four  species  of  Lagena. 

E    £    2 


420        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 


the  teeth  of  these  microscopic  and  apparently  useless  organs — 
useless  at  any  rate  so  far  as  their  generic  and  specific  characters 
are  concerned,  for  what  can  it  matter  to  the  sponge  whether  the 
number  of  the  teeth  be  three  or  more  or  less,  or  whether  the  teeth 
at  the  two  ends  of  the  spicule  be  equal  or  unequal  ?  Yet,  in  the 
course  of  evolution,  such  characters  as  these  have  become  more 


FIG.  187. — Siliceous  Spicules  (Chelse)  of  Sponges.      (After  Ridley  and 
Dendy,  in  "  Challenger  "  Report.) 

A.  A',  front  and  side  views  of  chela  of  Esperella  lapidiformis,  x  360. 

B.  B',  front  and  side  views  of  chela  of  Esperiopsis pulchella,  x  284. 

C.  C',  front  and  side  views  of  chela  of  Cladorhiza  (?)  tridentata,  x  360. 

or  less  fixed  and  constant  and  they  are  evidently  handed  down 
from  generation  to  generation  by  the  ordinary  process  of  heredity.1 
We  pointed  out  in  an  earlier  chapter  that  the  external  form 
of  the  entire  sponge  is,  in  some  cases  at  any  rate,  explicable 
as  an  adaptation  to  peculiar  conditions  of  the  environment. 
We  saw  this  is  the  case  of  the  curious  "  Crinorhiza  "  form 
(Fig.  168)  which  prevents  the  sponge  from  sinking  into  the  soft 
mud  or  ooze  on  which  it  rests.  Let  us  glance  for  a  moment, 
however,  at  another  deep  sea  sponge,  Esperiopsis  challengeri, 
dredged  up  by  the  "Challenger"  Expedition  from  a  depth  of 

1  The  reader  should  refer  back  to  Fig.  88  for  other  forms  01  sponge  spicules. 


NON-ADAPTIVE   CHAEACTEES 


421 


825  fathoms  in  the  Malay  Archipelago.  In  some  respects  this  is 
the  most  remarkable  sponge  that  has  ever  been  discovered.  Its 
form  (Fig.  188)  is  absolutely  unique  and  resembles  rather  that  of 
some  graceful  plant  than  those  of  other  sponges.  It  belongs  to  a 
group  of  sponges  whose  members  occur 
mostly  in  much  shallower  water  and  are  by 
no  means  distinguished  by  beauty  or  sym- 
metry of  shape.  In  spiculation,  moreover, 
and  other  minute  anatomical  features,  it 
exhibits  no  striking  peculiarities  ;  indeed,  so 
closely  does  it  agree  with  more  ordinary 
species  of  Esperiopsis  that  it  has  not  as 
yet  been  considered  necessary  to  separate  it 
generically.  How  then  can  we  account  for 
this  wonderful  form  ?  Can  we  say  that  it 
is  an  adaptation  to  any  special  conditions  of 
the  environment  ?  It  hardly  seems  likely 
that  this  is  the  case,  for  we  know  that  the 
relatives  of  this  sponge  get  on  well  enough 
with  all  sorts  of  other  forms,  for  the  most 
part  more  or  less  irregular  and  ill-defined, 
and  we  know  of  nothing  in  the  conditions 
under  whichit  lives  to  make  such  a  unique  and 
beautiful  form  especially  advantageous.  The 
"  Challenger"  obtained  no  less  than  thirteen 
specimens  of  this  sponge  at  the  same  place, 
and  the  form  appears  to  be  quite  constant. 
No  doubt  the  undisturbed  condition  of  the 
at  great  depths  favours  symmetry  of 


FIG.  188.—  Esperi^sis 
chailengeri,  X  £• 
(After  Kidley  and 
Bendy,  in  "Chal- 
lenger" Eeport.) 


sea 

growth  in  sessile  organisms  like  sponges, 
but  why  this  particular  and  absolutely  unique 
shape,  so  different  from  anything  else  that 
has  ever  been  met  with  ? 

These  are  questions  which  we  cannot 
answer,  and  it  must  suffice  to  point  out  that  such  cases  can  hardly 
be  explained  by  the  theory  of  natural  selection.  Many  factors  must 
combine  in  determining  the  course  of  evolution  of  any  particular 
organism,  and  some  of  the  characters  which  result  from  their 
interaction  may  perhaps  have  no  more  direct  relation  to  the 
necessities  of  the  organism  in  the  struggle  for  existence  than  has 
the  colour  of  a  pebble  to  its  continued  existence  on  the  sea-shore. 


422       OUTLINES   OF   E VOLUTION ABY  BIOLOGY 

It  may  even  be  questioned  whether  any  large  proportion  of 
specific  characters  can  have  arisen  through  the  action  of  natural 
selection.  The  characters  by  which  we  are  accustomed  to  sepa- 
rate one  species  from  another — such  as  minute  differences  in 
size,  shape  and  colour — are  usually  so  slight  that  we  are  hardly 
justified  in  attributing  to  them  any  adaptive  value.  They  may, 
however,  form  the  starting  points  from  which,  under  the  influ- 
ence of  natural  selection  and  use  and  disuse,  adaptive  characters 
may  subsequently  arise. 


The  application  of  the  principles  of  organic  evolution  to  the 
problem  of  the  origin  and  progress  of  the  human  race  cannot  be 
adequately  dealt  with  in  the  present  volume,  while  at  the  same 
time  we  cannot  altogether  ignore  it,  for  not  only  is  it  in  man 
himself  that  we  find  the  most  remarkable  illustration  of  what 
has  been  accomplished  by  evolution,  but  the  future  progress  of 
mankind  must  depend  in  large  measure  upon  the  correct 
understanding  of  the  principles  in  question. 

Buffon,  more  than  a  century  ago,  pointed  out  the  close  resem- 
blance in  anatomical  structure  between  man  and  the  higher 
apes,  and  it  is  clear  that  both  he  and  Lamarck  were  only  pre- 
vented by  religious,  scjaiplea^from  definitely  maintaining  the 
origin  of  the  human  species  from  ape-like  ancestors,  a  view 
which  at  the  present  time  is  universally  accepted  amongst 
scientific  men.  This  reluctance  to  admit  the  obviously  close 
relationship  of  the  human  species  with  the  apes  was  one  of  the 
evil  results  of  the  intellectual  dishonesty  and  obscurantism  of 
the  middle  ages.  If  we  go  back  to  the  days  of  Carthage,  we 
find  that  the  explorer  Han  no  did  not  hesitate  to  speak  of  the 
"  gorillas  "x  which  he  met  with  in  Africa  as  hairy  men  and 
women  of  the  woods,  and  although  this  was  doubtless  going  too 
far  in  the  opposite  direction,  it  shows  not  only  that  he  recognized 
the  relationship  but  also  that  he  approached  the  question  with  a 
mind  entirely  free  from  prejudice. 

Man  is  one  of  the  latest  products  of  organic  evolution,  and  his 
appearance  upon  the  scene  possibly  does  not  date  further  back  than 
Pliocene  times.  It  is  said  that  flint  flakes  of  human  workman- 
ship have  been  discovered  in  early  Pliocene  deposits  of  Burmah, 

1  Probably  really  chimpanzees. 


ANTIQUITY  OF   MAN  423 

but  perhaps  the  earliest  actually  human  fossil  so  far  known  is 
a  lower  jaw  of  very  massive  form  which  was  found  in  a 
deposit  of  probably  early  Pleistocene  age  near  Heidelberg,  and 
described  by  Schoetensack  in  1908  under  the  name  Homo 
heidelbergensis.  This  name  implies  that  in  the  opinion  of  its 
author  the  fossil  man  of  Heidelberg  was  generically,  but  not 
specifically,  identical  with  the  human  beings  which  now  exist. 
The  question  of  the  distinction  of  species  in  the  genus  Homo, 
however,  is,  as  in  most  other  genera,  a  very  difficult  one,  and 
opinions  are  divided  as  to  whether  only  one  or  several  species 
should  be  recognized  amongst  the  existing  races  of  mankind. 

So  close  is  the  anatomical  agreement  between  the  genus 
Homo  and  the  higher  apes  that  there  is  little  room  for  con- 
necting links  between  them,  the  difficulty  being  rather  to  find 
any  definite  characters  by  which  they  can  be  separated  than  to 
discover  reasons  for  bringing  them  together.  Nevertheless,  the 
gap,  small  as  it  is,  has  been  filled  by  the  discovery,  by  Dr. 
Dubois  in  1894,  of  the  remains  of  a  semi-human,  ape-like 
creature,  to  which  the  name  Pithecanthropus  erectus  has  been 
given.  These  remains  were  found  in  Pliocene  strata  in  the 
island  of  Java  and  consist  of  a  portion  of  a  cranium,  a  thigh 
bone  and  two  molar  teeth.1 

In  deposits  of  Pleistocene  age  undoubtedly  human  remains 
become  fairly  abundant,  and  are  found  associated  with  the  bones 
of  other  mammals,  of  which  many,  such  as  the  mammoth,  the 
cave  bear  and  the  woolly  rhinoceros,  are  now  extinct. 

The  chief  factors  which  contributed  towards  the  gradual 
transformation  of  ape-like  into  human  creatures  were  doubtless 
the  same  as  those  which  have  operated  in  the  evolution  of  other 
branches  of  the  animal  kingdom,  namely,  the  efforts  which  the 
ancestral  forms  were  obliged  to  make  in  order  to  maintain  them- 
selves in  the  struggle  for  existence  and  the  natural  selection  of 
favourable  variations.  In  no  group  of  the  animal  kingdom  do  we 
see  better  illustrated  the  importance  of  Lamarck's  principle  of 
the  effect  of  changed  habits  upon  bodily  organization.2 

The    anthropoid   ancestors  of    man    were  undoubtedly,  like 

1  Some  authorities  regard  these  remains  as  truly  human.     The  difference  of 
opinion  is  itself  very  instructive. 

2  Darwin,  notwithstanding  what  he  says  in  the  sixth  edition  of  the  "  Origin  of 
Species  "  about  the  effects  of  use  and  disuse  (quoted  on  p.  392),  denies,  in  the 
"  Descent  of  Man  "  (Ed.  2,  p.  619),  that  man  has  risen  through  his  own  exertions. 
I  can  see  no  reason  for  such  a  pessimistic  view. 


424        OUTLINES   OF    EVOLUTIONARY   BIOLOGY 

existing  Simiidse,  arboreal  in  habit.  Their  limbs,  whose  primi- 
tive pentad actyl  structure  indicates  their  origin  from  some 
little-specialized  mammalian  type,  had  ceased  to  be  used  exclu- 
sively as  organs  of  locomotion  on  the  ground  and  become  adapted 
for  climbing  trees.  Hence  the  opposable  toe  and  thumb,  which 
enabled  their  possessor  to  obtain  a  firm  grasp  of  the  branches. 
In  the  existing  apes  hand  and  foot  are  very  similar,  and.  both, 
as  regards  function,  partake  as  much  of  the  nature  of  hand  as  of 
foot,  whence  the  name  "  Quadrumana  "  applied  to  the  group  by 
the  older  naturalists.  In  many  of  the  lower  apes  or  monkeys  a 
long  prehensile  tail,  which  can  be  twisted  round  the  branches,  is 
of  great  assistance  to  the  animals  in  their  arboreal  habits,  but 
in  the  higher  forms,  such  as  the  chimpanzee,  the  gorilla  and 
the  orang  utan,  the  tail  has  already  disappeared. 

These  tailless  forms  are  still  mainly  arboreal,  but  when  they 
have  occasion  to  come  to  the  ground  they  assume  a  semi-erect 
posture  in  locomotion.  In  walking  they  usually  put  their  hands 
to  the  ground,  but  generally  resting  upon  the  backs  of  the  fingers, 
instead  of  upon  the  palms  as  in  the  lower  apes.1  Hence  in  these 
large  old-world  apes  the  hands  and  feet  are  more  completely 
differentiated  from  one  another. 

The  next  step  in  the  evolution  of  man  was  probably  the 
gradual  abandoning  of  arboreal  habits  and  the  assumption  of  a 
more  completely  erect  attitude  during  locomotion.  This  appears 
to  have  been  the  real  starting  point  of  his  human  career.  It 
was  this  change  of  habit  which  led  to  those  structural  modifica- 
tions required  to  balance  the  body  properly  in  its  new  position, 
and  to  the  further  differentiation  between  hand  and  foot,  the 
latter  being  used  to  the  complete  exclusion  of  the  former  as  an 
organ  of  locomotion  and  at  the  same  time  losing  its  prehensile 
character,  so  that  one  of  the  chief  distinguishing  features  between 
man  and  the  higher  apes  is  that  in  man  the  great  toe  has  ceased 
to  be  opposable. 

In  this  way  the  hand  was  completely  set  at  liberty  for  the 
purposes  of  a  prehensile  organ.  It  was,  however,  no  longer  used, 
as  in  the  apes,  chiefly  for  laying  hold  of  branches  in  climbing 
and  for  conveying  food  to  the  mouth,  but  more  for  grasping 
loose  objects  and  converting  them  into  weapons  or  tools  for 
different  purposes.  Hanno  observed  that  the  anthropoid  apes 
which  he  met  with  in  Africa  defended  themselves  with  stones, 

1   Vide  Beddard,  "  Mammalia  "  (Cambridge  Natural  History,  Vol.  X),  p.  570. 


EVOLUTION    OF   MAN  425 

so  that  we  can  hardly  say  that  the  use  of  tools  or  weapons  is  an 
exclusively  human  attribute ;  but  the  hand  of  man  is  undoubtedly 
one  of  his  most  characteristic  features,  and  by  its  aid  man  has 
been  able  to  make  for  himself  an  unlimited  number  of  what  are 
really  additional  organs,  derived  not  from  his  own  body  but  from 
his  environment. 

To  the  experience  gained  by  the  exercise  of  the  hands  in  so 
many  different  ways  must  also  be  attributed  in  large  measure 
the  extraordinary  mental  and  moral  development  which, more  than 
anything  else,  separates  mankind  from  the  apes.  The  constant 
exploitation  of  the  environment  stimulated  and  exercised  the  brain, 
which  in  turn  suggested  methods  of  employing  the  hands  and 
the  tools  which  they  had  constructed  to  ever  greater  advantage ; 
and  thus  both  hand  and  brain  progressed  until  they  attained  their 
present  wonderful  state  of  efficiency. 

The  development  of  the  brain  has,  however,  long  since  taken 
the  lead  in  human  evolution  and,  considering  the  immense 
difference  in  intellectual  capacity,  it  is  surprising  that  there 
should  be  so  little  structural  difference  between  the  brain  of 
man  and  that  of  the  higher  apes.  As  Huxley  has  pointed 
out: — 

"  It  is  only  in  minor  characters,  such  as  the  greater  excavation 
of  the  anterior  lobes,  the  constant  presence  of  fissures  usually 
absent  in  man,  and  the  different  disposition  and  proportions  of 
some  convolutions,  that  the  Chimpanzee's  or  the  Orang's  brain 
can  be  structurally  distinguished  from  Man's." l 

Intellectual  capacity  appears,  however,  to  depend  mainly  upon 
the  size  of  the  cerebral  hemispheres,  which  is  doubtless  correlated 
with  the  number  of  nerve  cells  present,  and  in  this  respect  the 
human  brain  is  far  in  advance  of  that  of  any  ape. 

One  of  the  most  important  characters  which  differentiate  man- 
kind from  the  apes  is  the  faculty  of  articulate  speech,  but  even  this 
undoubtedly  had  its  beginnings  in  the  inarticulate  sounds  made 
by  ape-like  ancestors,  either  as  spontaneous  expressions  of  their 
emotions  or  as  a  means  of  communicating  more  or  less  definite 
ideas  to  one  another.  The  development  of  speech  provided  a 
new  means  for  the  transmission  of  experiences  from  one  genera- 
tion to  the  next,  and  as  a  consequence  knowledge  began  to 
accumulate  in  the  minds  of  the  human  race.  When  oral  tradition 

1  Huxley,  "  Man's  Place  in  Nature,"  p.  140  (Collected  Essays,  Vol.  VII). 


426        OUTLINES   OF   EVOLUTIONARY   BIOLOGY 

gave  place  to  the  establishment  of  written  records,  and  methods 
were  invented  for  the  indefinite  multiplication  of  these,  the 
accumulation  of  knowledge  took  place  at  a  much  more  rapid  rate 
and  it  became  possible  for  every  human  being,  at  any  rate  in 
civilized  communities,  to  benefit  from  the  experience  of  all  his 
fellow  men.  The  acceleration  of  intellectual  and  moral  progress 
which  has  been  brought  about  in  this  way  has  led  to  results 
which  may  well  have  deluded  man  into  the  belief  that  he  is  the 
centre  of  the  universe  and  that  between  himself  and  the  lower 
animals  there  is  a  great  gulf  fixed. 

Man  has  indeed  acquired  a  degree  of  control  over  his  environ- 
ment and  over  his  own  destiny  which  distinguishes  him  from  any 
of  the  lower  animals,  but  at  the  same  time  the  conditions  of  his 
life  have  become  far  more  complex  and  the  young,  at  any  rate  in 
civilized  communities,  have  to  go  through  a  long  course  of 
education  before  they  are  fit  to  enter  upon  the  struggle  for 
existence  on  their  own  account.  Amongst  the  lower  animals  all, 
or  almost  all,  the  faculties  necessary  for  existence  are  directly 
inherited  from  the  parents,  incorporated  in  the  organism  itself, 
but  man  inherits  in  this  way  only  a  relatively  small  proportion 
of  the  powers  which  he  requires  to  carry  on  his  life.  The  greater 
part  of  human  experience  is  of  too  recent  origin  to  have  become 
heritable ;  it  has  to  be  acquired  afresh  by  education  in  every 
generation,  and  in  this  respect  is  strikingly  contrasted  with  the 
instincts  of  the  lower  animals. 

The  immense  advance  which  civilized  man  has  made  since  he 
parted  company  with  his  ape-like  progenitors  is  shown,  not  only 
by  the  fact  that  he  has  already  to  a  large  extent  subjugated  the 
remainder  of  the  organic  world  and  directed  the  forces  of  inanimate 
nature  into  new  channels  to  serve  his  own  purposes,  but  also  by 
the  intelligent  forethought  which  he  exercises  for  the  future 
welfare  of  his  own  race.  At  the  present  time  this  forethought 
is  being  exercised  in  new  directions,  and  a  determined  effort  is 
being  made  to  apply  our  knowledge  of  the  principles  of  organic 
evolution  to  the  furtherance  of  human  progress.  In  spite 
of  many  differences  of  opinion,  such  as  that  which  still 
prevails  with  regard  to  the  relative  importance  of  breeding 
and  education,  we  have  undoubtedly  arrived  already  at  many 
results  which  are  of  vital  significance  in  this  connection, 
and  the  intelligent  application  of  scientific  principles  must 
here,  as  always,  lead  to  further  progress.  We  cannot,  however, 


HUMAN   PROGRESS  -427 

control  the  future  of  the  human  race  until  we  have  familiarized 
ourselves  with  the  past,  and  learned  to  recognize  the  part 
played  by  the  numerous  different  factors  which  have  been  con- 
cerned in  the  evolution,  not  only  of  mankind,  but  of  the  whole 
organic  world. 

It  must  always  be  remembered  that  the  problem  before  us  is 
one  of  extreme  complexity,  and  that  we  cannot  afford  to  neglect 
any  of  the  factors  involved.  Above  all,  we  must  avoid  dogma- 
tizing on  an  insufficient  basis  of  fact.  If,  for  example,  the  very 
modern  doctrine  of  the  non-inheritance  of  acquired  characters  is 
allowed  to  influence  the  actions  of  men  and  women,  and  if,  after 
all,  this  doctrine  should  prove  to  be  erroneous,  as  seems  highly 
probable  at  the  present  time,  the  attempt  to  apply  biological 
principles  to  the  welfare  of  humanity  may  well  end  in  disaster. 
The  penalty  which  each  generation  has  to  pay,  in  regard  to 
bodily  and  mental  organization,  for  the  mistakes  and  misfortunes 
of  its  ancestors,  may,  in  most  cases,  be  a  very  small  one ;  but  if 
there  is  any  penalty  at  all,  and  if  the  mistakes  are  continued 
from  generation  to  generation,  it  will  surely  be  a  cumulative  one. 

In  dealing  with  problems  of  this  kind  a  rational  conservatism, 
with  a  mind  always  open  to  conviction,  seems  the  only  safe 
attitude  to  adopt. 


INDEX 


ABBREVIATION  of  ontogeny,  273. 
a  biogenesis,  214 — 216. 
Acacia  pycnantha,  seedling,  Fig.  132. 
acacias,  life-history,  280. 
Acanthodes,  291,  Fig.  138.^ 
accessory  chromosomes,  135. 
accessory  idioplasm,  167. 
accretion,  20. 
achromatic  figure,  74. 
acquired    characters      and      human 
progress,  427. 
Bordageon,182. 
,,         Brewer  on,  178, 

180. 
,,  ,,        Brown  -Sequaid 

on,  180. 
Buff  on  on,  367, 

368. 
,.  ..         Charles  Darwin 

on,  180,  393. 
.,  ,,        de     Vries     on, 

414. 
.,  ,,        Eigenmann  on, 

183. 

t>  ,,        Erasmus     Dar- 

win on,  372. 

„  „        Henslowon,183 

,,  ,,        Herbert  Spencer 

on,  185. 

„  ,,        Hilgard  on,  178. 

,,  inheritance  of, 
165, 169, 176— 
193,  404. 

,.  ,,        Lamarck        on, 

379,  380,  382. 

„  ,,        meaning          of 

term,  156, 176, 
177. 

„  ,,        Sumner  on,  182. 

,,        Weismann     on, 

176,  179. 

actinula  larva,  1 22. 
activity  of  male,  84,  85. 
adaptation,  235,  365. 

,,          and  design,  212. 
,,  ,,   fluctuating      varia- 

-   tion,  414,  415. 


adaptation  and  mutation,  414,  415. 
,,  „     natural       selection, 

396,  404,  405. 
,,          average,  184. 
,,          co-operation  of  factors  in 

bringing  about,  405. 
,,          embryonic,  267 
,,          Erasmus     Darwin      on, 

372,  373. 

,,          in  animals,  334 — 349. 
,,          individual,  184. 

in  inorganic  world,  395. 
in  leaves  of  acacias,  280. 
in  plants,  350—363. 
mutual,  414,  415. 
origin  of,  183. 
resulting    from  survival 

of  fittest,  391. 
„          Eobert     Chambers     on, 

383,  384. 

adipose  tissue,  56,  Fig.  19. 
aesthetic  development  of  man,   in- 
fluenced by  insects,  364. 
aesthetic  sense,  in  courtship,  349. 
affinities,  natural,  228,  229. 
after-effects,  191. 
age  of  habitable  earth,  285—287. 
age  of  ocean,  287. 
aggregate  species,  224. 
aggressive  resemblance,  337. 
Agnatha,  290. 
Agnostus  princeps,  Fig.  134. 
air-bladders  of  Fucus,  99. 
air-breathing     vertebrates,     origin , 

256. 

ala  spuria,  305. 
albumen,  23. 
alcoholism,  157. 
alimentary  canal,  124. 
allantois,  270. 
allelomorphs,  200,  206. 
aloes,  350. 

Alpine  plants,  181,  262,  350. 
Alps,  Arctic  vegetation  on,  333. 
alternation  of  generations,  101. 
in  aphides,  144. 
in  ccelenterates,  121. 
in  ferns,  101—106. 
in  flowering  plants,  106—112. 


430 


INDEX 


Amauris  niavius,  346,  Fig.  177. 
Amblyopsis,  183. 

ambulatory  appendages    of   arthro- 
pods, 246,  247. 

ambulatory  legs  of  vertebrates,  235. 
Ameghiuo,  253. 
amitotic  nuclear  division,  80,  Fig.  36. 

in  Copromonas,  83. 
amnion,  270. 
Amoeba,  12—21,  Fig.  2. 
,,         mitosis  in,  79. 
„         sensitive  to  stimuli,  188. 
Amphibia,  geological  range,  284. 
„          limbs,  235. 
origin,  292. 

amphimixis,  146,  171,  Fig.  78. 
,.          and  variation,  173. 
Amphioxus,  adult,  266,  267,  Fig.  118. 
„  early  development,  45 — 

48,  265,  Fig.  13. 
„          free  swimming  embryo, 

271. 

,,          ovum,  140. 
Amphisbsenidse,  250. 
Amphitherium,  302. 
anabolism,  9,  Fig.  1. 
analogy,  235,  247. 
anatomy,  evidence  afforded  by,  232 

—262. 

ancestral  history,  229. 
Anchitherium,  310. 
An  con  sheep,  154. 
Andalusian  fowls,  blue,  200,  201. 
Andrews,  C.  W.,  312. 
andrcecium,  106. 
anemophilous  flowers,  111,  353. 
Angnisfragilis,  250,  Fig.  105. 
animals,  compared  with  plants,  34, 

35. 
„         dependent  on  green  plants, 

35. 

anisogamous  conjugation,  84. 
anisogamy  in  Fucus,  101. 
Anomodontia,  296. 
Antarctic  continent,  328. 
anteaters,  333. 

Antedon,  hybridization  in,  174,  175 
„         recapitulation  in  develop- 
ment, 273,  275,  Figs.  123, 
124. 
antheridia,  of  Fern,  103, 104,  Fig.  51. 

„  Fucus,  100,  Fig.  47. 
antheridial  cell  of  pollen  grains,  108, 

Fig.  54. 
anthers,  107. 

anticryptic  colouration,  337. 
antiquity  of  man,  422,  423. 


antlers,  125. 
apatetic  colours,  336. 
apes,  Buffon  on,  367,  368. 

and  man,  Buffon  on,  370. 
„       „      Lamarck  on,  382. 
„       „      relationship  of,  422— 

425. 

aphides,  parthenogenesis  in,  143. 
Aphrodite,  story  of,  214. 
apogamy  in  ferns,  105. 
aposeuiatic  colouration,  342. 
Apteryx,  258,  Figs.  Ill,  112. 
Apus,  dispersal  of,  327. 
arborescent  colonies,  40. 
Archseopteryx,  300,  305—307,   Fig. 

151. 
archegonium,    of    Fern,    103,    104, 

Fig.  52. 
„  „     flowering    plant, 

108. 

architype,  232. 

Arctic  climate,  retreat  of,  333. 
„      vegetation  on  Alps  and  Pyre- 
nees, 333. 

areas  of  distribution,  319,  320,  330. 
Argus  pheasant,  349. 
Aristotle,  163,  385. 
arm,  of  man,  237,  238,  Fig.  94. 
armadillos,  333. 
arrow  worm,  origin  of  germ  cells, 

130. 

Artemia,  chromosomes,  73. 
Arthropoda,  limbs,  246,  247. 
arthropods  and  vertebrates,  255. 
artificially  produced  characters,  1 56, 

157. 
artificial  parthenogenesis,  145. 

„        selection,   395,   396,   410— 

413. 
„  „         and  fertility,  418, 

419. 

artiodactyl  limbs,  241,  Fig.  98. 
Arum,  fertilization,  355. 
Ascaris,  chromosomes  in,  73. 

,,       distinction  between  somatic 

and  germ  cells,  166. 
„       fertilization     of    egg,     131, 

Fig.  64. 

,,       mitosis,  79. 
,,       origin  of  germ  cells,  129. 
ascidian,  aotult,  277,  Fig.  129. 
,,         degeneration  in,  400. 
larva,  277,  Fig.  130. 
asexual  reproduction,  in   Eudorina, 
91. 

Hydra, 


116. 


INDEX 


481 


asexual    reproduction,     in     Obelia, 

119. 
,,  inPandorina, 

89. 

Aspidium,  Figs.  48,  50. 
asters,  in  mitosis,  71. 
atavism,  261,  262. 
attraction  of  gametes,  143. 

„          ,,         ,,         in   Coccidium, 

141,  142, 

„         „         ,,         in    ferns    and 
mosses,  141, 
142. 
in    Spirogyra, 

142. 

„  sexes,  125—127. 
,,        sphere,  71. 
Aucuba,   male  and    female    plants, 

111. 

Aurelia,  meristic  variation  in,  149. 
Australia,  fauna  and  flora  of,  250, 

328,  329,  331. 
Australian   climate,   adaptation    to, 

280. 

automatism,  19. 
Avebury,  Lord,  281. 
average  adaptation,  184. 
axial  filament,  of  spermatozoon,  140. 
Axoniderma  mirabile,  Fig.  168. 

B. 

BABIRTJSA,  tusks,  406,  Fig.  185. 
baboons,  Buffon  on,  367,  368. 
Bacillus  saccobranchi,  66,  Fig.  27. 
Bacteria,  217,  218,  Fig.  83. 

„        alleged  spontaneous  genera- 
tion, 215. 

„        origin,  219. 

„        structure,  66,  Fig.  27. 
ISadhamia,  Fig.  29. 
Bakewell,  387. 

Italcena  mysticetus,  245,  Fii*.  101 . 
Balsenidee,  teeth,  260. 
barnacles,  degeneration,  401. 
barriers  to  migration,  320. 
basal  cell,  48. 
bast,  65. 

Bates,  H.  W.,  346. 
Batesian  mimicry,  346. 
Bateson,  W.,  195,  205,  207,  208. 
bats,  301. 
„     dispersal,  323. 
,,     wings,  236,  243,  Fig.  99. 
battle,  Charles  Darwin  on  Jaw  of,  388. 
,,      Erasmus  Darwin  on  law  of,  373. 
beech,  evergreen,  329. 


beech,  seedling,  281,  Fig.  133. 
bees,   alleged    spontaneous  genera- 
tion, 214,  215,  343. 
,,     as  pollen  carriers,  354,  355,  &c. 
„     parthenogenesis,  144. 
,,     proboscis,  354,  Fig.  179. 
beetles,  mutation  in,  159. 
Behring  Strait,  former  land  connec- 
tion, 309. 
,,  ,,      former  mild  climate 

328. 

Bell,  340. 

Biffen,  E.  H.,  205. 
binomial  nomenclature,  226. 
biogenesis,  216. 
biogenetic  law,  265. 
Biology,  origin  of  name,  373. 

„        science  of,  1. 
Biophoridae,  219. 
biophors,  22,  66,  168,  219.  • 
birds,  compared  with  reptiles,  305. 
dispersal,  323,  324. 
egg,  140,  Fig.  70. 
evolution,  306,  307. 
geological  range,  284. 
limbs,  235. 
origin,  299,  300. 
wing,  243,  244,  Fig.  99. 
blackberries,  white,  204. 
blastocoal,  47. 
blastoderm,  270. 
blastogenic  characters,  176. 

„  ,,          inheritance, 

169. 
„  „          origin,      159, 

160. 

,,  variations,  149, 157 — 160. 

blastomeres,  45,  79. 
blastopore,  47,  265,  269. 
blastosphere,  45, 265. 
blastostyles,  120. 
blastula*,  45,  265. 

,,       interpretation,  279. 

„       of     Amphioxus,     45,     47, 

Fig.  13. 

of  frog,  268,  Fig.  119.     • 
„        of  Hydra,  118,  Fig.  59. 
blind  worm,  250,  Fig.  105. 
blood,  51,  Fig.  15. 
blue  Andalusian  fowls,  200,  201. 
boa  constrictor,  dispersal,  325. 
Bodo,  life-history,  85—87,  Fig.  38. 
body  cavity,  48,  124. 
body  wall,  124. 
ttombus    terrestris,    robbing    clover, 

355,  356. 
bone,  57. 


432 


INDEX 


bony  fishes,  292. 

Bordage,  E.,  182. 

Boveri,  T.,  146,  174. 

brachiopods,  289. 

brain,  in  evolution  of  man.  425. 

„      of  man  and  apes,  425. 
branching      classification,      Charles 

Darwin  on,  387. 
„  evolutionary  series,  La- 

marck on,  376. 

Branchiosaurus,  293,  Fig.  139. 
Branchipus,  dispersal,  327. 
breathing,  8. 
breeding  experiments  with  Papilio, 

348. 

Brewer,  178. 

brimstone  moth,  larva,  Fig.  169. 
brittle  star,  275,  Fig.  126. 
Brontosaurus,  299,  Fig.  144. 
Browu-Sequard,  180. 
Buceros  rhinoceros,  Fig.  407. 
budding,  263. 

„        in  Hydra,  116. 

in  Obelia,  119,  120. 
Buffon,  on  sterility  of  species  when 

crossed,  418. 
views  of,  365—370. 
Burbank,  L.,  204. 
Butler,  Samuel,  3,  186,  191,  369. 
Biitschli,  O.,  19,  21,  22. 
butterflies,   warning  colours,  343 — 

347. 

butterfly,  leaf,  338,  Fig.  171. 
,,        metamorphosis,  264. 
„        proboscis,  354. 

0. 

CACTI,  350. 

,,      thornless,  204. 
csenogenetic  characters,  278 — 281. 
Camolestes,  301. 
Caesar's  horse,  262. 
Cainozoic  Era,  284,  285. 
calcareous  skeletons,  26. 
Calceolaria,  distribution  of,  328. 
calcium,  influence  upon  segmenta- 
tion, 45. 

callosities,  Buffon  on,  367,  368. 
calyx,  106. 
cambium,  76. 
Cambrian  Epoch,  284. 
camel,  Buffon  on,  367. 

„      feet,  241,  Fig.  98. 
Camelo-pardalis,  Lamarck  on,  379. 
cancer,  nature  of,  408. 
Cancer  phalangiuin,  339. 


candle,  analogy  of,  2,  5. 

Candollea  gramimfolia,  fertilization 

362. 
cane  sugar,  attracting  spermatozoa 

141. 

canine  teeth,  reversion  in,  262. 
cannon  bone,  241. 
Capsella  bursa-pftstoris,  development 

48,  Fig.  14. 
carbohydrates,  27. 
carbon,  6. 

carbon  dioxide,  6,  8. 
Carboniferous  Epoch,  284. 
Carchesium,  40. 
carnivores,  301. 
carpals,  237. 
carpels,  106. 
Oarpophaga  chathamensis,  416. 

„  novas  zealandice,  416. 

carrion  flies,  355. 
cartilage,  56,  Fig.  20. 
casein,  23. 

castration,  effect,  125. 
Catasetum,  fertilization,  362. 
caterpillar,  larval  organs,  275. 

stick,  337,  338. 
cats,  inheritance  of  mutilation,  178. 

,,    stump-tailed,  179. 
caudicle,  361. 
cave-animals,    bleaching    inherited 

183. 
cell,  12. 

body,  13. 
definition  of,  38. 
division,  69—80. 

„       limits  of,  81. 
history  of  term,  36,  37. 
membrane,  6C 
plate,  74. 
sap,  62 
theory,  38. 

,,       limitations  of,  65. 
typical,  69,  70,  Fig.  30. 
wall,  27,  28,  69. 
cellular  structure,  36. 
cellulose,  27,  28. 
centrosome,  71. 

,,          absence  in  higher  plants, 

141. 

„  »       ,,  ovum,  141. 

,,          as  stimulus  to  develop- 
ment, 146. 
,,  iu    spermatozoon,     139. 

Fig.  69. 

,,  in  unfertilized  egg,  146. 

,,  origin  of,  in  zygote,  141. 

centrosphere,  71. 


INDEX 


433 


Cepnalaspis,  290,  Fig.  135. 
Ceratodus  (see  Neoceratodus). 
cercariae,  144. 
cereals,  fertilization,  412. 

,,       improvement,  410—413. 
,,       Mendelian    inheritance    in, 

205. 
cervical    vertebrae,   of    giraffe,  248, 

Fig.  104. 
„    whale,    248, 
318,  Fig.  103. 
Cetacea,  evolution  of,  314 — 318. 

limbs,  236. 

Chaetopterus,      artificial     partheno- 
genesis, 145. 
chalk,  26. 

chamaeleon,  colour  change  in,  341. 
Chambers,  Robert,   views  of,  383 — 

385. 

change  of  function,  255,  256,  262. 
Chara,  gametes,  141. 
characters,  compound,  208. 
"  Chatham,"  H.M.S.,  399. 
Chatham  Islands,  325,  398—400,  416, 

417. 

chelae,  of  sponges,  420,  Fig.  187. 
Chelonia,  295. 
chemical  affinity,  6,  7. 
,,        processes,  9. 
„         stimulus    to    development, 

146. 

chemically  modified  larvae,  157. 
chemotaxis,  141, 143,  189. 
chick,  embryo,  265,  Figs.  117,  122. 
chimpanzee,  brain,  425. 

Buffon  on,  370. 

Chinese  women,  small  feet,  156. 
Chironomus,  paedogenesis,  144. 
Chlamydomonas,  40. 
chlorophyll,  7,  8,  28. 
cells,  64. 

,,  corpuscles,  32,  64. 

chloroplastids,  32,  64. 

,,  in  Spirogyra,  96. 

Chordata,  277. 
chordate  condition,  26G. 
chromatin,  70,  71. 

,,  in  heredity,  166, 169,  174, 

206. 

chromatophores,  in  Spirogyra,  96. 
chromomeres,  73,  168,  206. 
chromosomes,  71,  73,  168. 

„  and  sex,  135,  Fig.  67. 

„  differential     division, 

171,  Fig.  77. 

„  in  gametophyte,  138. 

,,  „    sporophyte,  139. 


L. 


chromosomes,  integral  division,  171, 

Fig.  77. 
„  maternaland  paternal, 

137,  207,  Fig.  68. 
pairing,  133,  137,  206, 

Fig.  68. 

reduction,    132,     133, 
137,   138,  206,  207, 
Fig.  68. 
chrysalis,  264. 
cilia,  39. 

Gloria  intestinalis,  Fig.  129. 
circumcision,  179,  180. 
Cirolana,  227. 

cirripedes,  degeneration,  401. 
Cladocera,  parthenogenesis,  144 
Cladorhiza  longipinna,  Fig.  168. 

„          (?)    tridentata,      spicules, 

Fig.  187. 
classes,  226. 
classification,  225—228. 

and    phylogeny,    230, 

Fig.  87. 
artificial,  228. 
Buffon  on,  366. 
Charles     Darwin     on, 

387. 

Lamarck  on,  374,  375. 
natural,  228,  Fig.  86. 
clavicle,  234,  Figs.  89,  90,  93. 
clear- winged  moths,  mimicry,  343, 

Fig.  175. 

cleavage  of  ovum,  75. 
climate,  adaptation  to,  280. 
,,        changes  in,  327. 
„        influence  of,  Buffon  on,  366. 
,,  „        „   Lamarck      on, 

376. 

Miocene,  328. 

clover  leaf,  meristic  variation,  149. 
coal,  energy  in,  4. 

Coccidium,  gametes,  87,  88,  Fig.  39 
,,         maturation  of  ovum,  139. 
coccyx,  in  man,  261. 
cockroaches,  dispersal  of,  325. 
Codonella,  Fig.  9. 
Cceciliidse,  250,  Fig.  106. 
Ccelenterata,  114. 
ccelenterate  and  gastrula,  265,  269, 

270,  277. 

ccelom,  48,  124,  266. 
ccelomate    and    ccelenterate    types, 

124,  Fig.  62. 
coelomic  epithelium,  124. 

,,        pouches,  266. 
cold-blooded  animals,  6. 
colloids,  23. 

F  F 


434 


INDEX 


colony  formation  in  Hydra,  116. 
„  Obelia,  119. 

,,  ,,          ,,  Protozoa,  40 — 44. 

colour  changes  in  animals,  341,  342. 
colours  of  animals,  336,  337. 

,,       ,,         ,,        Erasmus  Darwin 

on,  372. 

,,       ,,  flowers,  355. 
column,  of  flower,  360,  363. 
combustion,  energy  liberated  by,  4, 6. 

,,  nature  of,  2,  5,  6. 

communication  between  cells,  187. 
compound  characters,  208. 
conceptacles  of  Fucus,  100. 
Condylarthra,  309,  313. 
cone  of  attraction  (or  reception)  in 

Goccidium,  88,  143. 
congenital  variations(characters),157. 
Conilera,  227. 
conjugants,    in     Paramoecium,     92, 

Fig.  41. 
conjugation  of  chromosomes,      137, 

Fig.  68. 

„  gametes,  33,  82-85, 
131,  141, 
206,  207, 
Fig.  65. 

M  in     Ascaris, 

131,   Fig. 
64. 
„  ,,       in  Bodo,  87, 

Fig.  38. 

„  ,,       in        Oocci- 

dium,  88, 
Fig.  39. 

„  „       in      Coelo- 

mata,  125. 

,,  ,,       in  Copromo- 

nas,      83, 
Fig.  37. 

„  ,,       inEudorina, 

91,  Fig.  40. 

,,  ,,       in  ferns,  105. 

',,  ,,       in  flowering 

plants, 
109. 

,,  ,,       in       Fucus, 

101,   Fig. 
47. 

,,  ,,       in      Hsema- 

tococcus, 

33,  Fig.  5. 

,,  „       in  heredity, 

171. 

„  ,,       in      Pando- 

rina,     89, 
Fig.  10. 


con  j  ugation  of  gametes  in  Para- 
mcecium, 
92,  93, 
Fig.  41. 

,,  ,,       in     Spiro- 

gyra,  96, 
97,  142, 
Figs.  43, 
44. 


nium,  97. 
,,          of    nuclei    in    zygote, 

131,  Fig.  64. 
,,         origin  of,  127. 

results  of,  84,  87. 
connecting    links,    232—235,   305— 

318. 
„  „        destruction      of, 

222. 

conodonts,  289. 
conservatism    of    germ    cells,    184, 

190. 

continental  islands,  fauna,  332. 
continents,  permanence,  329. 
continuity  of  germ  plasm,  166,  168, 

Fig.  76. 
„  life,  67. 

„  ,,  parent  and  offspring, 

Erasmus  Darwin  on, 
370. 

,,  ,,  protoplasm,  66. 

continuous  selection,  411. 

,,  variations,  148,  150. 

contractile  tissue  (see  muscle  fibres). 

,,  vacuole,  14,  18. 

convergence,  247  —  255. 

,,  in  Acacia,  280. 

„  „  flightless  birds,  397. 

„  plants,  2fi2. 
,,  „  proboscis  of  bees  and 

Lepidoptera,  354. 
"  Convolvulus  major,"  fertilization, 

351,  352. 
co-operation,  40. 

,,  of  factors  in  evolution, 

421. 

Oopromonas,  life-history,  83,  Fig.  37. 
,,  nuclear  reduction,  139. 

coracoid,  234,  Figs.  89,  93. 

„         vestigial,  257,  Fig.  90. 
corals,  114,  265. 

cork,  cellular  structure,  37,  Fig.  6. 
corolla,  106. 
corpora  lutea,  126. 
corpuscles,  blood,  51,  5?,  Fig.  15. 
correlation,  409,  415. 
cortical  tissues,  50. 


INDEX 


435 


cosmopolitan  distribution,  320. 

Cosmozoa,  216. 

cotyledons,  caenogenetic  characters, 

280,  281,  Figs.  56,  133. 
cowslip,  fertilization,  356. 
crab,  larva,  276,  Fig.  128. 

„     swimming,  247,  Fig.  102. 
creation,  Lamarck  on,  375,  376. 

special,  212—214,  Fi«r.  82. 
,,  „       Buffon  on,  369. 

,,  „       Linnaeus  on,  222. 

Creodoutia,  318. 
Crepidula,  323. 
Cretaceous  Epoch,  284. 
Crinoidea,  274. 

Crinorhiza  form,  335,  420,  Fig.  168. 
Cristatetta    mucedo,   statoblast,   Fig. 

165. 

Crocodilia,  295. 
cross-fertilization  of  flowers,  351 — 

363. 

crossing  (see  hybridization). 
Crustacea,    fresh    water,    dispersal, 

327. 

gills,  255. 

cryptic  colouration,  337. 
crystalloids,  23. 

currents,  dispersal  by,  321— 3'25. 
curve  of  frequency  of  error,  153. 
„     „  variation,  150, 153,  Figs.  72, 

73. 

Cuscuta  europcea,  402,  403,  Fig.  184. 
cushion  plants,  262,  350. 
Cyathaspis,  290. 
cyclopean  larvae,  157. 
Cyclostomata,  289. 
cyst,  20,  83. 
cytology,  67. 
cytolysis  of  ovum,  147. 
cytoplasm,  13,  69. 

in  heredity,  174,  175. 
cytostome,  83. 
cytotaxis,  142. 
cytotropism,  142. 

of  gametes,  143. 
Cyttaria,  329. 
Cyttarocyclis,  Fig.  9. 


D. 


DARBISHIKE,  A.  D.,  197. 

]>arwin,  Charles,  views  of,  164 — 167, 
176,  180,  185,  195,  208,  222— 
224,  329,  351,— 353,  356,  357, 
359,  385—388,  391—393,  410, 
415,  423. 


Darwin,  Erasmus,  views  of,  370 — 373. 

Francis,  192. 
death,  21,  162,  168. 
De  Candolle,  386. 
decay,  nature  of,  4. 
deep  sea  animals,  adaptation  in,  335, 

336. 

deer,  feet,  241,  Fig.  98. 
degeneration,  397 — 403. 

,,  in  ascidiaus,  400. 

,,  ,,  flightless  birds,  397. 

,,  ,,  Morioris,  400. 

results  of,  398. 
Delage,  Y.,  146. 

Delphinus  delphis,  Figs.  161,  162, 
Democritus,  369. 
dendrons,  59. 
dentition  of  dog  and  thylacine,  251 — 

253,  Figs.  107,  108. 
denudation,  285,  288. 
dermatogen,  50. 
Descartes,  11,  260. 
descent  with  modification,  221. 
design,  doctrine  of,  212. 
Desmacidonidae,  spicules,   419,  420, 

Fig.  187. 
determinants,  167,  206. 

,,  effect  of  stimuli  upon, 

,189-. 
,,  vibrations  in,  189. 

deutoplasm,  140,  267. 
development,  263—281. 

,,          and  unconscious  memory, 

192. 
.,          Erasmus     Darwin      on, 

371,  372. 

.,          factors  in,  192. 
,,          of  birds  and  reptiles,  270. 
„          of  flowering  plants,  48 — 

50,  109,  Fig.  14. 
„          of  Frog,  268—270,   272, 

Figs.  119,  121. 

,.          of  Hydra,  118,  Fig.  59. 
Devonian  Epoch,  284. 
De  Vries.  Hugo,  154,  204,  224,  410— 

414. 

Dianthus,  353. 
diatoms,  25. 
dichogamy,  353. 
Dictyocysta,  Fig.  9. 
Didelphyidse,  301. 
differences     between     plants     and 

animals,  34,  35. 
differential  division  of  chromosomes, 

177,  Fig.  77. 

differentiation,  39,  43,  44,  60,  119. 
„  sexual,  85. 

FF2 


486 


INDEX 


diffusion  of  gases,  8. 
digestion,  in  Amoeba,  16. 
digestive  cavity,  of  Hydra,  115. 
,,  ,,        „  Medusa,  120. 

„  Obelia,  119. 
digits,  reduction,  239 — 241. 
dihybridism,  201-203,  Figs.  80,  81. 
dimorphic  flowers,  356. 
dimorphism  of  gametes,  139. 
Dinosauria,  297. 
Dinotherium,  314,  Pig.  160, 
dioecious,  99,  105. 
Diplodocus,  299. 
Diplosoma    cryatallinum,  larva,  Fig. 

130. 

Dipnoi,  255,  291. 
Diptera,  p?edogenesis,  199. 
direct  nuclear  division,  80,  Fig.  36. 
discontinuity  between   species,    222, 

Fig.  85. 
„  in  distribution,  319, 320, 

333. 
„  in  evolutionary  series, 

Lamarck  on,  376. 
„  in  organic  world,  221. 

discontinuous  variation,  149,    153 — 

156. 

Dismorphia  praxinoe,  345,  Fig.  176. 
dispersal  of  organisms,  320—327. 
distribution,  geographical,  319 — 333. 
disuse  (see  use  and  disuse). 

„       of  wings,  effects  of,  397,  398. 
divergence  in  evolution,  213. 

A.  E.  Wallace  on,  390. 
,,          Charles  Darwin  on,  387. 
division  of  labour,  39,  43,  44,  60,  85, 

119. 
,,  ,,-•  between  male  and 

female,  127. 
dodder,  402,  403,  Fig.  184. 

, ,      vestigial  leaves,  262,  Fig.  184. 
dodo,  258,  397. 
dog,  skull  and    dentition,  251 — 253, 

Figs.  107,  108. 
„    vestigial  teeth,  260. 
dogfish,  embryo,  270,  Fig.  120. 
dolphin,  Figs.  161,  162. 
dolphins,  convergence  in,  248. 
,,        shark-toothed,  318. 
domestication,  Charles  Darwin    on, 

410. 

,,  Lamarck  on,  378. 

dominant  characters,  197,  207. 
Draba  verna,  elementary  species  in, 

224,  412. 

drone  flies,  mimicking  bees,  343. 
„         mistaken  for  bees,  215. 


Dubois,  E.,  423. 

Dujardin,  38. 

dynamics  of  cell-division,  74. 

E. 

ECHIDNA,  234,  301,  Fig.  92. 
echinoids,  larval  stage,  276. 
Echinus,  blastpmeres,  45. 

,,       hybridization  in,  174. 
ectoderm,  47. 

„        of  coelomates,  124. 

„  Hydra,  115,  116,  118. 
edentates,  301,  333. 
education,  426. 
eels,  dispersal  of,  325,  326. 
egg  cell  (see  ovum). 
,,    of  Ascaris,  mitosis,  79,  Fig.  35. 
„    „  bird,  140,  Fig.  70. 
„    „  frog,  268. 
eggs,  dispersal  of  fish,  322. 

„     similarity     in     different    or- 
ganisms, 162. 
,,     size,  267. 
Eigenmanu,  183. 
Elasmobranchii,  291. 
electrical  energy,  transmission,  189. 
electric  eel,  256. 

,,       organs,  256. 
electro-magnetic  theory  of  mitosis, 

74,  75. 

elementary  species,  224,  412. 
elephants,  ancestry,  312 — 314,   Fi». 

159. 

„          limbs,  239. 
Elephas,  314. 
elite,  411. 

Elodea,  evolution  of  oxygen,  31. 
embryo,  fixation,  126. 

of  chick,  265,  Figs.  117,  122. 
„  Fucus,  101,  Fig.  47. 
„          ,,  mammals,  270. 

,,  rabbit,  265,  Fig.  117. 
embryology,   Erasmus    Darwin   on, 

371,  372. 
„  evidence  afforded    by, 

261—281. 
embryonic  cell,  48. 
embryo-sac,  48,  106,  107,  108. 
embryos  of  birds  and  reptiles,  270. 
Emily  Henderson,  sweet  pea,  208. 
Empedocles,  369. 
Encrinites,  274. 
endoderm,  47. 

„          of  coelomates,  124. 
„          of  Hydra,  115,  116,  118. 
endoplasm,  14,  22. 
endosarc,  14. 


INDEX 


437 


endosperm,  109. 
energy,  conservation  of,  6. 
„      manifested  in  life,  4,  5. 
,,       of  chemical  affinity,  7. 
,,       source  of,  4,  6,  7,  9. 
,,  ,,       ,,    in  green  plants,  30. 

engrains,  186. 
entelechy,  11. 
enteron,  47,  265. 

„        in  Hydra,  115. 
entomophilous  flowers,  111,  353. 
enucleate    eggs,     fertilization     (see 

merogony). 
environment,  control  of,  426. 

„  influence  of,  5,  6,  166, 

182,  183. 

A.E.Wallace  on, 390. 

Buffon  on,  366,  367. 

Charles  Darwin  on, 

387,  392. 

Lamarck  on,   377 — 
379. 

upon    development, 
193. 

„     germ       plasm, 
159. 

Eocene  Epoch,  284. 
Eohippus,  309,  310,  Fig.  154. 
Ephydatiafluviatilis,  gemmules,  Fig. 

166. 
epiblast,  47,  48,  266. 

of  Hydra,  115,  118. 
epicoracoid,  234,  Figs.  89,  93. 
epidermis,  54,  Fig.  17. 

„         of  plants,  50,  64,  Fig.  26. 
epigamic  ornamentation,  336,  349. 
epigenesis,  163. 
Epihippus,  310. 
epilepsy,  in  guinea  pigs,  1 80. 
episematic  colouration,  342. 
Epistylis,  40. 

epithelium,  54,  Figs.  16—18,  28. 
epochs  of  earth's  history,  284,  285. 
equatorial  plate,  72. 
Equidse,  pedigree  of,  307 — 312. 
Equus,  309,  311,  Fig.  153. 
eras  of  earth's  history,  284,  285. 
Esperella  lapidiformis,  spicules,  Fig. 

187. 
Esperiopsis  challengeri,  420,  421,  Fig. 

188. 
,,         pukhella,    spicules,    Fig. 

187. 
eucalypts,  331. 

leaves,  280. 

Eudendrium,    migration     of     germ 
cells,  123. 


Eudorina,  42,  90,  Fig.  40. 
Eurypteridae,  291,  Fig.  137. 

,,  size  of,  405. 

Eutheria,  301. 

„         first  appearance  of,  302. 
evolution,  factors  of,  365 — 427. 
in  development,  163. 
individual,  263. 
of  sex,  81—147. 
progressive,  192,  334. 
theory  and    evidence  of, 

211—364. 
,,          versus    special     creation, 

212—214,  Fig.  82. 

ex-conjugants,  in  Paramcecium,  93. 
excretion,  9. 

,,        in  Amoeba,  17. 
experiments  in  heredity,   174,   175, 

194—209. 

explosive  character  of  living  mole- 
cule, 18. 
extinction  of  groups,  231. 

,,         ,,  species,  Buff  on  on,  366 
extracted  dominants,  198. 
,,        recessives,  198. 
eye-colour,  Mendelian  inheritance  of, 

205. 
eyes,  pineal,  258 — 260. 

F. 

FACTORS,  co-operation  of,  208. 
„         in  development,  192. 
„          ,,  germ  plasm,  206,  207. 
,,         of  organic  evolution,  365 — 

427. 

,,         permutations  and  combina- 
tions of,  206. 
faeces,  10. 

families,  in  classification,  226. 
Farmer,  J.  B.,  206. 
fat,  56,  Fig.  19. 

feather  star,  hybridization,  174,  175. 
'  „         „      recapitulation  in  life- 
history,       273 — 275, 
Figs.  123,  124. 
feathers,  acquisition  of,  177. 
female  animal,  114. 

characters,  84,  85,  127. 
„      dependence  on  male,  127. 
femur,  237. 
fermentation,  218. 
fern,  life-history,  101—105,  Figs.  48— 

52. 

ferns,  dispersal,  321. 
fertility  and  cross-fertilization,  352, 

357. 
„        test  of  specific  identity,  418. 


488 


INDEX 


fertilization,  adaptation  of  flowers  for, 

351—363. 
„          chemical    stimulus    in, 

146. 
0,  development      without, 

145. 

in  heredity,  171. 
membrane,  147. 
of  flowers,  110,  351  — 

363. 

5,     ovum,  85,  Fig.  65. 
„         „    in  Ascaris,  131, 

Fig.  64. 
.,          5,         „     „  Ccelomata, 

125. 

„          „        „     „  ferns,  105. 
„  „         „     „  Fucus,  101. 

„  ,,         „     „  Hydra,  118. 

,,          ,,         „     „  medus8e,121. 
iibrillar    structure    of    protoplasm, 

22. 

fibula,  237. 
fig-wort,  353. 
filament  of  stamen,  107. 
finger,    inheritance    of    mutilation, 

180. 
First   Cause,   Erasmus  Darwin  on, 

372. 

fish  eggs,  dispersal,  322. 
fishes,  bony,  292. 
„      deep  sea,  336. 
,,      geological  range,  284. 
fish- like  stage  in  ontogeny  and  evo- 
lution, 272,  273,  277,  278,  279. 
fission,  in  Amoeba,  21. 
„       „  Bodo,  87. 
,,       ,,  Copromonas,  83. 
„       „  Hsematococcus,  29. 
fixation  of  embryo  in  uterus,  126. 
flagella,  in  Bodo',  87. 
„        „  Eudorina,  91. 
,,        ,,  Haeinatococcus,  29. 
flame,  nature  of,  2. 
Flemming,  69. 
flightless  birds,  258. 

„  ,,     and  fluctuating  varia- 

tion, 415. 
,,  ,,       ,,    natural  selection, 

397,398. 
flint,  24. 

floating  islands,  dispersal  by,  324. 
flower,  structure  of,  106,  107,  Fig.  53. 
flowering  plants,  life-history,   106 — 

112. 
flowers,  adaptation  for  fertilization, 

351—363. 
„        sexual  characters,  110,  111. 


fluctuating  variations,  148,  150,  155, 
Figs.  72, 73. 

,,  :,         and    adapta- 

tion,     414, 
415. 

l?  .,         and      natural 

selection, 
414. 
^  ,,        De  Yries  on, 

413,  414. 

flukes,  parthenogenesis  in,  144. 
foam  structure  of  protoplasm,  21. 
foetal  membranes,  270,  278. 
foetus,  in  Mammalia,  270. 
food  materials  of  animals,  16. 
„  ,,         „  green  plants,  30- 

32. 

„    nature  of,  7. 

,,    vacuoies,  14. 

food-yolk,  267,  278. 

„          influence  of,  268,  271. 
foot,  artiodactyl,  241. 
„     of  apes  and  monkeys,  424. 
„     „  camel,  241,  Fi~.  98. 
,,     „  deer,  241,  Fig.^S. 
,,     „  elephant,  239. 
,,     ,,  hippopotamus,  241,  Fig.  98. 
„     ,,  horse,  241,  Fig.  97. 
,,     ,,      ,,      atavism  in,  262. 
,,     „      „      evolution  of,  310—312, 

Figs.  154—158. 
„     „  Litopterna,     253—255,    Fig. 

109. 

„     „  llama,  241. 
,,     „  man,  238,  Fig.  94. 
,,     ,,  oxen,  241. 

„  pig,  241,  Fig.  98. 

,,  rhinoceros,  241. 

„  seal,  244,  Fig.  100. 

,,  sheep,  241. 

,,  tapir,  241,  Fig.  96. 

,,  ungulates,    239—241,    253— 

255. 

,,     pentadactyl,  238. 
,,     perissodactyl,  241. 
Foraminifera,  26,  Fig.  4. 
fossilization,  287,  288. 
fowls,  Mendelian  inheritance  in,  200, 

201. 

foxgloves,  mutation  in,  154. 
Francotte,  135. 
Freia,  Fig.  9. 

frequency  of  error,  curve,  153. 
fresh  water  animals,  dispersal,  325, 

326. 

frog,  early  development,   268—270, 
Fig.  119. 


INDEX 


480 


frog,  life-history,  272,  Fig.  121. 

,,  ,,  interpretation    of, 

279,  Fig.  131. 

, ,    pineal  eye  in,  260,  Figs.  115,116. 
frogs  and  toads,  293. 
fruit  trees,  propagation,  205. 
fruits,  dispersal,  321. 
Fuchsia,  distribution,  328. 
Fucus,  99—101,  Figs.  45—47. 
Functions  of  organisms,  <">. 
Fundulus,     cyclopean     larvae,    157, 

Fig.  75. 
Fungi,  34,  321. 
funiculus,  of  ovule,  108. 
furze,  recapitulation  in  seedlings,  280. 

G. 
Galaxias    nigothoruk,     dispersal    of, 

326. 

Gallardo,  75. 
Galton,  Francis,  209. 
Galton's  law  of  inheritance,  209. 
Galton's  polygon,  155. 
Galtonia,  mitosis  in,  76,  Figs.  33,  34. 
gametes,  33,  83  (see  also  germ  cells). 
„        attraction  of,  141,  142,  143. 
.,        conjugation  of,  207. 
„       evolution  of  male  and  female, 

84,  85. 
of  Bbdo,  87. 
„        „  fern,  104. 
,,         „  flowering  plant,  108,  109, 

110. 

„  Fucus,  100. 
,,        ,,  Pandorina,  89. 

„   Spirogyra,   96,    97,    142, 

143. 

„  Volvox,  91. 
purity  of,  200,  206. 
„        sexual  dimorphism  of,  113, 

139. 

gametic  nuclei  in  Paramcecium,  93. 
garnetogenesis,  132,  Fig.  65. 
gametophyte,  101. 

„    *        chromosomes  of,  138 
of  fern,  103,  Fig.  50. 
,,  ,,  flowering  plant,  107, 

108. 
suppression     of, 

112. 

gamobium,  121. 
ganglion,  59. 
ganoids,  292. 
gastroea,  277. 
gastral  cavity,  47. 

,,      in  Hydra,  115,  118. 
„       in  Obelia,  119. 


gastrula,  47,  265,  277. 

,,        interpretation  of,  265,  279. 
of  Hydra,  118. 
„  Sagitta,  130,  Fig.  63. 
gastrulation,  in  birds   and   reptiles, 

270. 

„    frog,  269,  Fig.  119. 
Geikie,  Sir  A.,  285. 
gemmules,  dispersal  of,  327. 

,,          in  pangenesis,  164.r- 

,,          of    fresh    water    spdnge, 

327,  Fig.  166.        \ 
genera,  225. 
generative  cells,  10^. 
Genesis,  Book  of,  212. 
genital  ducts,  125. 
geographical  di^tribv  don,  319—333. 
,,  „       summary,  330. 

,,  isolation,  416,  417. 

geological  formations,  284. 

„          history  of  the  earth,  284, 

285. 

„         periods,  :284. 
,,          range   of  animal  groups, 

284. 

record,  283,  287—304. 
geometer   moths,    caterpillars,    337, 

338,  Fig.  169. 

Geoplana  exulans,  dispersal,  325. 
Geotria,  distribution,  329. 
germ  cells  (see  also  gametes). 

,,  and  somatic  cells,  97,  98t 
99,  113,  129,  166,  167, 
168. 

„          conservatism,  184,  190. 
immortality,  162,  168. 
,,          independence,  99. 
,,          maturation,  138. 
„          migration   in    Hydrozoa, 

123. 

,,          origin  in  Ascaris,  129. 
,,  „       Cceloniates,  124, 

130. 

„       Hydrozoa,  123. 
,,  ,,      plants,  130. 

,,       Sagitta,  130. 
,,          potentialities,  163. 
,,          sensitive  to  stimuli,  188. 
germinal  disk,  140. 

,,        selection,  173. 
,,         variations,  149,  157—160. 
germination  of  fern  spore,  103,  Fig. 

49. 
„  pollen      grain,     108, 

Fig.  54. 

seed,  109,  Fig.  56.   * 
germ  layer  theory,  48. 


440 


INDEX 


germ  plasm,  complexity,  167. 

„  composition,  172,  Fig.  78. 

„         'constitution  205,206. 
„  continuity,      166,     168, 

Fig.  76. 

„          influenced  by  environ- 
ment, 159. 
gestation  of  nature,  R.  Chambers  on, 

384. 

gigantic  animals,  303,  304,  405 — 409. 
Gila  monster,  342. 
gill  slits,  in  Amphioxus,  266. 

embryos,  261,  273,  Fig. 

122. 

gills  of  crustaceans  and  fishes,  255. 
.giraffe,  248,  Fig.  104. 

,,      Lamarck  on,  379. 
glacial  periods,  327,  328. 
glass  snakes,  250. 
gliadin,  23. 
glucose,  28,  31. 
glutinin,  23. 
Godlewski,  174. 
gonads,  113. 

,,        in  Ooelomates,  124* 
,,         ,,  Hydrozoa,  124. 
,,         ,,  medusae,  121. 
gonoducts,  125. 
gonophores,  122. 
gonotheca,  120. 
"gorillas,"  Hanno  on,  422. 
gorse,  recapitulation  in  seedlings, 280. 
gradation  in  nature,  Buffon  on,  366. 

„  structure,  232. 
,,       of  animals,   Lamarck    on, 

376. 
Grantia  compressa,  larva,   322,  Fig. 

164. 
grape  hyacinth,  curve  of  variation, 

150,  Fig.  72. 
„      sugar,  28,  31. 
grasshopper,    spermatogenesis,    134, 

Fig.  66. 
Gray,  Asa,  386. 
Gray,  J.  E.,  227. 
grazing  mechanism,  309. 
Great  Britain,  a  continental  island, 

332. 

Greenland,  former  mild  climate,  328. 
Grew,  Neheiniah,  37. 
gristle,  56. 
growth,  10,  20. 

control  of,  408. 

in  Amoeba,  20. 

„  animal  tissues,  75. 

„  plants,  76. 

, ,        , ,      periodicity  of ,  1 9 1 . 


guard  cells,  64,  Fig.  26. 
guinea  pigs,  Brown- Sequard's  experi- 
ments, 180. 

Gulf  Stream,  dispersal  by,  322,  323. 
Gulick,  417,  418. 
gut  wall,  124. 
Gymnotus,  256. 
gynoecium,  106. 

H. 

HABIT,  of  plants    modified  by  en- 
vironment, 181. 
habitat,  319, 
habits,  adaptation  in,  337. 

„      influence    of,    Lamarck    on, 

377,  378,  379,  380. 
,,      in  plants,  191. 
Haeckel,  Ernst,  66,  229,  265,  277. 
hsematids,  52,  Fig.  15. 
hsematochrome,  28. 
Haematococcus,  27 — 34,  Fig.  5. 
,,  conjugation,  89. 

dispersal,  327. 
haemoglobin,  53. 
hair,  vestigial,  261. 
hairs  of  Tradescantia,  61,  Fig.  25. 
hand  of  man,  256,  424,  425. 

,,      „  monkeys  and  apes,  424. 
Hanno,  422,  424. 
harmony,  in  colouration,  336. 
Harpax  tricolor,  339. 
Hatteria  (see  Sphenodon). 
Hawaiki,  399. 

heat,  in  living  organisms,  6. 
Heidelberg,  fossil  man,  423. 
Heilprin,  A.,  319. 
Heliconinae,  345. 
Heliconius  ethilla,  345,  Fig.  176. 
Heloiierma  suspectum,  342. 
Hemiphaga  chathamensis,  416. 

,,          novce  zealandi a1,  416. 
Henslow,  G.,  183. 
Herbst,  45. 

heredity,  161 — 210  (see  also  inherit- 
ance). 

„         Buffon  on,  369. 
,,         Charles    Darwin's   theory, 

164—166. 
,,        fertilization     experiments, 

174,  175. 

Galton  on,  209,  210. 
,,         in  neuter  insects,  189,  190. 
„  Protista,  161. 
Lamarck  on,  380,  381. 
„         Mendelian       experiments, 
194—209. 


INDEX 


441 


heredity,  mnemic  theory,   186    191, 

192. 

,,        nature  of  problem,  163. 
,,         Pearson  on,  210. 
,,         Weismann's  theory,  166 — 

174. 

Hering,  E.,  186. 

hermaphrodite,  103,  105,  113,  114. 
hermaphroditism,  93,  116,  125,  402. 
heterogeny,    144. 
Heteroinita,     life-history,     85 — 87, 

Fig.  38. 

heterosporous  ferns,  105. 
heterostyled  flowers,  356,  Fig.  180. 
heterozygote,  207. 
hexadactylism,  154,  413. 
Hilgard,  E.  W.,  178. 
Hipparion,  311. 
Hippocampus  antiquorum,  341,   Fig. 

173. 

Hippopotamus,  feet,  241,  Fig.  98. 
histological  differentiation,  48. 
histology,  51. 
His,  Wilhelm,  178. 
hock,  of  horse,  240,  Fig.  95. 
holophytic  nutrition,  34. 
holozoic  nutrition,  34. 
Homo  heidelbergensis,  423. 
Homo,  species  of,  423. 
homologous  chromosomes,  137,  206, 

207. 

homology,  235,  247. 
homoplasy,  235,  247. 
homosporous  ferns,  106. 
homozygote,  207. 
honey  guide,  362. 
honey  sucking  apparatus,  354. 
Hooke,  Kobert,  36. 
Hooker,  J.  D.,  385. 
hormones,  126,  188,  408. 
hornbill,  406,  Fig.  186. 
hornets,  colours  of,  342,  343. 
horns,  excessive  development,  406. 
horse,  evolution  of,  307—312,  Figs. 

153—158. 
horse's  feet  (see  foot  of  horse). 

„       skeleton,  Fig.  95. 
horses,  Mendelian inheritance  in,  205. 
horse-worm  (see  Ascaris). 
humble  bee,    robbing    clover,    355, 

356. 

humerus,  237. 
humming  birds,  as  pollen  carriers, 

353. 

„       distribution,  320. 
,,  ,,       epigamic       orna- 

mentation, 349. 


hyacinth,  mitosis  in,  76,  Figs.  33,  34. 
hybridization,  194—209. 

„  and  evolution,  209. 

„  and     mutation,     413, 

414. 

,,  in  cereals,  413. 

Hydra,  114-118,  265,  Figs.  57—59. 
„       dispersal,  327. 
, ,       suppression  of  inedusoid,  1 23. 
hydranth,  119. 
hydrocaulus,  119. 
hydrotheca,  119. 
Hydrozoa,  123. 
hyperphalangy,  244. 
hypertonic  solution,  producing  par- 
thenogenesis, 145. 
hypoblast,  47,  48. 

„         of  Amphioxus,  47,  266. 
„  Frog,  269. 
„  Hydra,  115,  118. 
hypostome,  116,  119. 
Hypsidse,  345. 
Hyracotherium,  310. 


ICEBERGS,  dispersal  by,  324. 
Ichthyopterygia        (Ichthyosauria), 

297,  Fig.  142. 
paddles,  236,  244. 
Ichthyosaurus  communis,  Fig.  142. 
idants,  168. 
identical  twins,  173. 
idioplasm,  167. 
Idoliim  diabolicum,  339. 
ids,  168,  206. 
igneous  rocks,  282. 
Iguanodon,  297,  Fig.  143. 
illegitimate  unions,  in  Primula,  357. 
immortality  of  germ  cells,  162,  168. 

„  Protista,  161. 
immutability  of  species, 
Buff  on  on,  369. 
Lamarck  on,  374. 
Linnaeus  on,  222. 
individual  adaptation,  184. 

,,         characters,     transmission 

of,  175. 

,,         variations,  150. 
individuality  of  cells,  68. 
inertia  of  germ,  cells,  190. 
inflorescences,  variation  in,  150. 
Infusoria,  40,  Fig.  41. 
inheritance  (see  heredity). 

„          of    acquired    characters, 

165,  176—193. 
„          „      mutilations,  178, 179, 


442 


INDEX 


insect  communities,  as  individuals, 

190. 

insectivores,  301. 

insects,  as  pollen  carriers,  353 — 355. 
,,       dispersal  of,  323. 
,,       primordial  germ  cells,  130. 

selection  by,  363. 
-    „       sex  determination,  135,  136, 

Fig.  67. 

,,       spermatogenesis,  135. 
,,       wings,  247. 
instincts,  adaptation  in,  337. 

„         origin  of,  184. 
integral    division    of    chromosome*, 

171,  Fig.  77. 
integration,  44. 
integuments  of  ovule,  108. 
intercellular  substance,  57,  123. 
interclavicle,  234,  Figs.  89,  93. 
intercrossing,  swamping  effects,  416. 
interstitial  cells,  116. 
intussusception,  20. 
invagination,  47. 
invertebrates,  dispersal,  325. 

„  geological  range,  284. 

Ipomcea  purpurea,  fertilization,  351, 

352. 

irritability,  18. 
irritable  structures  in  flowers,  362. 

363. 

Isoetes,  105. 
isogamy,  84,  89,  97. 
isolation,  331,  416-419. 

„         Lamarck  on,  380,  381. 
Ithomiinse,  345. 


JELLY-FISH,  114,  121,  265. 

Joly,  286. 

Jurassic  Epoch,  284. 

K 

KAKAPO,  397. 

Kallima  inac.hi*,  338,  Fig.  171 

kangaroos,  301. 

Kant,  178. 

karyogamy,  128. 

karyokinesis,  69 — 79,  Figs.  31 — 35, 

karyoplasm,  14,  69. 

karyosome,  70,  77. 

katabolism,  9,  Fig.  1. 

kea,  398. 

khaki  clothing,  342. 

kidney,  effect  of  removal,  156. 

kinetic  energy,  7. 

kingdoms,  226. 


kiwi,  258,  331,  397,  Figs.  Ill,  112. 
knee,  of  horse,  240,  Fig.  95. 

L. 

LABELLUM,  359. 
labyrinthodonts,  243,  406. 
Lagena,  non-adaptive  characters,419. 
Lamarck,  views  of,   176,    373—382, 

418.  ^ 

Lamarck's  four  laws,  382. 
Lamarckian  factors,  A.  R.  Wallace 

on,  393. 

,,  „         Charles  Darwin 

on,  166,  o92, 
393. 

neglect  of,  393. 
,,  ,,         Robert    Cham- 

bers on,  385. 

lampreys,  dispersal,  325,  326. 
,,          distribution,  329. 
,,         pineal  eye,  258,  260. 
lance- woods,  417. 
land  connections,  former,  328,  3'.9. 

,,    planarians,  dispersal,  325. 
larva  of  Antedon,  274. 

Ascidian,  277,  Fig.  130. 
crabs,  276,  Fig.  128. 
echinoids,  276. 
frog,  272,  Fig.  121. 
Grantia,  322,  Fig.  164. 
ophiuroids,  275,  Fig.  127. 
larvae,  272. 
larval  forms,  dispersal,  321,  322. 

„     organs,  272,  275,  278. 
Leach,  227. 
leaf  insects,  338,  339,  Figs.  170,  171. 

„   structure,  63-65,  Fig.  26. 
leg,  of  man,  237,  238,  Fig.  94. 
legitimate  unions,  in  Primula,  357. 
legumin,  23. 
Leigh,  G.  F.,  348. 
Lernuria,  328. 
lemurs,  distribution,  328. 
leopard,  distribution,  320. 
Lepidosiren,  255,  291. 
Leptinotarsa,  mutation  in,  159,  160. 
leucocytes,  51,  Fig.  15. 
level,  changes  of  in  land,  328. 
lianes,  350. 

life-history,  263  (see  also  ontogeny), 
life,  nature  of,  2,  3,  4,  11. 
Lilium,  germination  of  pollen  grain, 

Fig.  54. 
limbs,  of  arthrbpods,  246,  247. 

vertebrates,  235—246. 
Limnas  clirysippus,  346,  Fig.  177. 
linin  network,  70. 


INDEX 


443 


Liimsean  species,  412. 
Linnaeus,  222,  226,  228. 
Linnean  Society,  340,  385. 
lithium  larvae,  157. 
Lithobius,  host  of  Coccidium,  87. 
Litopterna,  253,  Fig.  109. 
lizards,  pineal  eye,  258. 

„       shoulder  girdle,  234,  Fig.  89.   i 
llama,  Buffon  on,  367. 

„     feet,  241. 
Lock,  B,  H.,  152. 
locomotion  in  Amoeba,  15. 
Loeb,  J.,  145,  146. 
Lubbock,  Sir  John,  281. 
Lull,  E.  S.,  307. 
luminous  organisms,  6. 
lung-fishes  (see  Dipnoi), 
lungs  and  swim-bladder,  255,  256. 
Lyell,  Sir  Charles,  324,  385. 
Lygosoma  dendyi,  417. 

,,         moco,  417. 
lyre  bird,  349,  Fig.  178. 
lysin  theory  of  fertilization,  146,  147. 

M. 

MACCULLOCH,  340. 
machine,  analogy  of,  2,  3,  10. 
Macropodia  rostrata,  339, 340,  Fig.  172. 
magnesium  larvee,  157,  Fig.  75. 
maize  root,  section,  37,  Fig.  7. 
male  animal,  1 14. 

,,f  characters,  84,  85,  127. 
malic  acid,  attracting   spermatozoa, 

141. 

Malthus,  386. 
Mammalia,  compared  with  Eeptilia, 

232—235. 
dispersal,  323. 
geological   history,    284, 

301—304. 
gigantic,  406. 
limbs,  235. 

nutrition  of  young,  270. 
origin,  300. 
ovum,  140,  Fig.  71. 
mammoth,  Buffon  on  extinction  of, 

366. 

man,     aesthetic     development     in- 
fluenced by  insects,  364. 
„     and  apes,  relations,  422,  423, 
424,  ^5. 
,,  ,,  ,,         Buffon  on, 

370. 

,,  ,,  ,,          Lamarck 

on,  382. 
„     antiquity,  284,  303,  422,  423. 


man,  control  of  environment,  426. 
,,     evolution,  422 — 427. 
„  „          Charles  Darwin  on, 

423. 
,,     influence  on  other  organisms, 

396. 

,,     limbs,  237,  Fig.  94. 
„     Mendelian  inheritance  in,  205. 
,,     progress  of,  422—427. . 
„     races  of,  423. 
,,     reversion  in,  262. 
,,     vestigial  hair,  261. 
taU,  261. 

Mantidsa,  338,  339. 
inanubrium,  120. 
Maoris  and  Morioris,  398 — 400. 
marginal  canal,  120. 
marine  animals,  dispersal,  321. 

„       fauna  and  flora,  323. 
Marsh,  O.  C.,  262,  307. 
marsh  tit,  distribution,  319,  320. 
Marsilea,  105. 
marsupial  mole,  253. 
„         wolf,  251. 
Marsupialia,  250,  301,  331. 

„  distribution,    329,   330, 

331,  332. 
extinct,  302,  332 
Mastodon,  314. 
Mastodonsaurus,  296. 
maternal  chromosomes,  137,  Fig.  68. 
„         functions,  a  handicap,  127 
matter,  indestructibility  of,  6. 
maturation  of  germ  cells,  132^  137, 

138,  Figs.  65,  68.     *—' 
in  heredity,  171. 
Mauritius,  258. 
Mediterranean  fauna,  323. 
medusae,  120,  121,  123,  Fig.  60. 
megagametes,  85,  88,  90. 
megalecithal,  267. 
meganucleus,  39,  92. 
megasporangia,  107. 
megaspores,  105,  106,  107,  108. 
Megatherium,  303. 
meiosis,  132,  206. 
memory,  186,  191,  192. 
Mendel,  G.  J.,  195. 
Mendelian  experiments,  194 — 209. 
,,  inheritance  in    Leptiiio- 

\  tarsa,  160. 

in  Primula,  356. 
,,  principles,  application  of, 

205. 

,,  proportions,  199. 

Mendelism,  195. 

„  and  mutations,  415. 


444 


INDEX 


Menura  superba,  349,  Fig.  178. 

meristem,  76. 

meristic  mutations,  154. 

,,         variations,  148,  149. 
merogony,  146,  174. 
Merychippus,  310,  Fig.  153. 
mesentery,  54. 
mesoblast,  48,  266. 
mesoblastic  somites,  266. 
mesoderm,  48,  124. 
mesogloea,  115,  116. 
Mesohippus,  310,  Figs.  153,  156. 
mesophyll,  50,  64,  Fig.  26. 
Mesozoic  Era,  284,  285. 
metabolism,  9,  Fig.  1. 
metacarpals,  238. 
metagenesis,  121. 
metameric  segmentation,   149,    266, 

279. 

metarn orphic  rocks,  283. 
metamorphosis,  263. 
Metaphyta,  44,  95. 
metapodials,  240,  241. 
metatarsals,  238. 
Metatheria,  301,  302. 
Metazoa,  44. 
mice,  dispersal,  325. 

,,     experiments  in  heredity,  179, 

182. 

microgametes,  85,  88,  91. 
microlecithal,  267. 
Microlestes,  301,  302. 
micronucleus,  39,  92. 
rnicropyle,  109. 
microsporangia,  107. 
microspores,  105,  106,  107. 
microzooids,  33. 
migration  from  north,  329. 
milk,  271. 
mimicry,  343—348. 

,,         and  fluctuating    variation, 
415. 

„        and  natural  selection,  396, 
405. 

„         rings,  345. 
mineral  salts,  23. 
Miocene  Epoch,  284. 
Miohippus,  310. 
mitosis,  69—79,  Figs.  31—35. 

,,      in  heredity,  169. 
mnemic  theory  of  heredity,  186,  191, 

192. 

moas,  258,  397. 
models  and  mimics,  345,  348. 
Mceritherium,  313,  Fig.  159. 
moles,  marsupial,  253. 
Molluscoida,  277. 


momentum  in  evolution,  406 — 409. 
monads,  83. 
Monera,  66. 
monoecious,  103,  105. 
monohybridism,  201,  Fig.  79. 
monopodial  branching   of  phyloge- 

netic  tree,  230,  231,  Fig.  87. 
monosome,  135. 

Monotremata,  234,296,  301,302,331. 
monstrosities,  154,  157. 
Morioris,  extermination,  398 — 400. 
mosses,  dispersal,  321. 
moths,  clear- winged,  343,  Fig.  175. 
,,      proboscis,  354. 
,,       protective  resemblance,  338 
motion,  in  living  organisms,  6. 
mountain  hare,  distribution,  333. 
mud,  dispersal  in,  327. 
mud-fishes,  255,  291. 
Miiller,  Fritz,  346. 
Miillerian  mimicry,  346. 
multicellular,  38. 

„  organisms,   origin  of, 

44. 

Multituberculata,  302. 
multiplication  of  cells,  69 — 80. 
MungoswelPs  wheat,  412. 
Muscari,  variation  in,  150. 
muscle  fibres,  57,  58,  116,  Figs.  21, 

22,  58. 

music  and  memory,  192. 
mutation  and  adaptation,  414,  415. 
,,  .,     evolution,  414. 

,,  .,     hybridization,         413 

414. 
,,          .,     Mendelism,  415. 

„     natural  selection,  414. 
theory,  224. 
mutations,  149,  153—156,  224. 
meristic,  154. 
origin,  159,  160. 

„       of    species    from, 

411,  412,  413. 
„          substantive,  154. 
mutilations,  inheritance  of,  178,  179. 
mutual  adaptation,  414,  415. 
Mycetozoa  (Myxoinycetes),  67,  Fig. 
29. 

N. 

NAGELI,  167. 

natural  affinities,  228,  229. 
„       selection,  395—409. 
,,  ,,         and  mutation,  414. 

A.  E.  Wallace  on, 

389— 391. 
Buffon  on.  368. 


INDEX 


445 


natural  selection,  Charles  Darwin  on, 

385—388,  392. 
,,  ,,         Erasmus       Darwin 

on,  373. 
,,  ,,         insufficiency        of, 

404. 

,,  ,,         insufficiency        of, 

Charles     Darwin 
on,  392. 
„  „         summary  of  theory, 

391.    ' 

,,       system,  226. 
nectaries,  359,  360,  414,  415. 
Neoceratodus,  255,  291,  Fig.  110. 
Neohipparion,  310,  Fig.  157. 
Nepenthes,  262. 
Nerocila,  227. 
nerve-cells,  58,  Figs.  23,  24. 

,,      fibres,  58. 
nerves,  59. 

nervous  system,  18,  121,  266. 
neurons,  58,  59. 

neuter  insects  and  heredity,  189, 190. 
newts,  294. 
New  Zealand,  fauna  and  flora,  258, 

328,  329,  331. 
nipples,  vestigial,  345. 
nitrifying  bacteria,  218. 
Nitrobacter,  218. 
Nitrosornonas,  218,  Fig.  84. 
nomenclature,  226,  227. 
non-adaptive  characters,  419 — 422. 
normal  curve  of  variation,  153. 

„       variations,  150. 
notochord,  266. 
Notornis,  397. 
Notoryctes,  253. 
nucellus,  108. 
nuclear  membrane,  69. 
nucleinic  acid,  71. 
nucleolus,  70,  77. 
nucleoplasin,  14,  22,  69. 
nucleus,  13,  69. 

,,         division    (see    mitosis    and 
amitotic  nuclear  division), 
in  heredity,  161,  166,  174. 
zygote,  83. 

numerical  variations,  149. 
nutrition,  7  (see  also  food  materials). 
„         of  embryo  in  Ccelomata, 
125.    " 

O. 

OBELIA,  119,  265,  Fig.  60. 

ocean,  age  of,  287. 

oceanic  islands,  fauna,  330,  332,  397. 

oceans,  permanence,  329. 


CEnothera,  mutntion  in,  155. 

oil-foam,  19,  21. 

Olenus  cataractes,  Fig.  134. 

Oligocene  Epoch,  284. 

ontogenetic    record,    obscuring    of, 

267,  278. 
ontogeny,  263. 

,,          abbreviation  of,  273. 

„         a  habit,  192. 

„         and  phylogeny,  265,  279, 

Fig.  131. 
Erasmus  Darwin  on,  371, 

372. 

,,         interpretation  of,  277. 
Onychophora,  distribution,  330. 
oocytes,  133. 
oogenesis,  133,  Fig.  65. 
oogonia,  of  Fucus,  1 00. 

„       in  oogenesis,  133. 
oospheres,  90,  100,  104,  108. 
ooze,  26,  283. 
opal,  24,  26. 
Ophisaurus,  250. 
Ophiura  ciliaris,  Fig.  126. 
ophiuroids,  275,  276. 
opossums,  301. 

opposable  great  toe  and  thumb,  424. 
orang,  brain,  425. 
"  orang  utan,"  Buffon  on,  370. 
Orchidaceue,  fertilization,  359—362. 
Orchis  mascula,   fertilization,   359 — 

362,  Fig.  182. 
orders,  226. 

Ordovician  Epoch,  284. 
organellse,  39. 
organic  evolution,  doctrine  of,  212 

(see  also  evolution). 
„       units,  38,  65. 
organism,  3. 

organisms,  first  appearance  of,  212. 
„  nature  of  first,  217,  219. 
„  number  and  variety  of, 

211. 

„         origin  of,  214—216. 
organizing  spirits,  A.  E.  Wallace  on, 

394. 

organs,  12,  38. 

origin  of  great  groups,  303,  304. 
„       „  living  things,  214 — 216. 
„       „  sex,  84,  85,  89,  126—128. 
Ornithorhynchus,     234,     260,    301, 

Fig.  91. 

Ornithosauria,  299. 
Ornithoscelida,  297. 
Orohippus,  310,  Figs.  153,  155. 
Osborn,  369. 
osmosis,  8,  17,  23. 


446 


INDEX 


osmotic  pressure,  effect  on  egg,  145. 
Ostracodermi,    289,    290,   Figs.   135, 

136. 

Otaria  hookeri,  paddles,  Fig.  100. 
otter  sheep,  154. 
ovary,  of  animals,  113,  116. 

„        ,,  flower,  107. 
oviducts,  125. 
ovotestis,  113. 

ovule,  48,  106,  107,  Fig.  55. 
ovum,  85. 

,,       interpretation,  279. 

„       maturation,  133,  Figs.  65,  68. 

,,       of  Amphioxus,  45,  Fig.  13. 
„  Ascaris,  131,  Fig.  64. 

„       „  bird,  140,  Fig.  70. 

„       „  Chara,  141. 

„  Coccidium,  88,  Fig.  39. 

,,       „  Eudorina,  91,  Fig.  40. 

„       „  fern,  104,  Fig.  52. 

„       ,,  flowering  plant,    108,  109, 
Fig.  55. 

.,      „  Fucus,  100,  Fig.  47. 

„       „  Hydra,  116,  Figs.  57,  59. 
,,  mammals,    140,    271,    Fig. 
71, 

,,      „  medusae,  121. 
„  plants,  141. 

,,       ,,  rabbit,  Fig.  71. 

„       „  sponges,  113. 
'    „       „  Volvox,  91,  Fi-.  11. 

„       segmentation  of,  263,  265. 

,,       similarity    in    different    or- 
ganisms, 162. 

,,       size  of,  267. 

„       typical,  140,  Fig.  69. 
oxen,  feet,  241. 
oxidation,  5. 
oxlip,  fertilization,  356. 
oxygen,  5,  6,  8,  31. 
oyster,  American,  introduced,  323. 

P. 

PADDLES,  236,  244,  247. 
pademelons,  colouration,  337. 
psedogenesis,  144. 
pairing  of  chromosomes,   133,    137, 

206,  Figs.  65,  66,  68. 
Palseohatteria,  295. 
Palseomastodon,  313,  Fig.  159. 
palaeontology,  evidence  afforded  by, 

287—304. 

Palaeospondylus,  289. 
Palaeozoic  Era,  284, 285. 
palingenetic  characters,  277. 
palms,  climbing,  350. 


Pandorina,  42,  89,  Fig.  10. 
paiigenesis,  164—166,  369,  Fig.  76. 
Papilio,      mimicry      in,     346 — 348, 

Fig.  177. 
parallel    modification    of    body  and 

germ  cells,  182. 

Paramcecium,  39,  98,  139,  Fig.  8. 
paraphyses,  of  Fucus,  100. 
parasites,  peculiarities  of,  401 — 403. 
parasitism,  of  gametophyte,  112. 
parenchyma,  65. 
Pariasaurus,  297,  Fig.  140. 
parietal  eyes,  258—260,  Figs.  114— 

116. 

„       foramen,  260. 
parthenogenesis,  143,  144. 

„  artificial,  144-147. 

Paruspalustris,  distribution,  319,320. 
passivity  of  female,  84,  85. 
Pasteur,  215. 

Patagonia,  beech  forests,  329. 
paternal  chromosomes,  137,  Fig.  68. 
peach  trees,  inheritance  of  acquired 

characters,;  182. 

peacock,  epigamic  ornamentation,349. 
Pearson,  Karl,  210. 
pea,  Sicilian,  208. 
„     structure    and    gern 

seed,  Fig.  56.,;      ^m 
peas,    Mendelian     experiments    on, 

196—199. 

pedigrees,  229,  311. 
Peebles,  Florence,  28. 
pelagic  organisms,  322,  33 
pentacrmoid  stage  of  Antedon,  274, 

275,  Fig.  124. 

Pentacrinus,  274,  Fig.  125. 
pentadactyl  limbs,  237,  294  (see  also 

limbs  and  feet) . 
periblem,  50. 

Pericopis  angulosa,  345,  Fig.  176. 
Peripatus,  Fig.  167. 

„          distribution,  330. 
pensarc,  119. 
perissodactyl,  241,  253. 
peristaltic  movement,  57. 
peritoneal  epithelium,  54,  Fig.  16. 
Permian  Epoch,  284. 
Perrhybris  malenka,  345,  Fig.  176. 
persons  in  colony,  119. 
Petalotricha,  Fig.  9. 
petals,  106. 

phagocytes,  phagocytosis,  52. 
phalangers,  301. 
phalanges,  238. 

Phascolotherium,  302,  Fig.   149. 
Phenacodus,  309,  Fig.  152. 


INDEX 


447 


phloem,  65. 

phosphorescent  organs,  336. 

phosphorus,  22,  71. 

photosynthesis,  30,  31. 

phyla,  226. 

phyllodes,  280, 

Phyllopteryx  eques,  341,  Fig.  174. 

phylogenetic  tree,  229. 

phylogeny,  229. 

,,          and     classification,    230, 
231,  Fig.  87. 

,.  ,,      ontogeny,  265,  279, 

Fig.  131. 

,,          Erasmus  Darwin  on,  372. 
physical  basis  of  life,  7. 

,,         conditions  of  earth,  changes 

in,  327. 
physiological  selection,  418. 

„  unit,  the  cell,  68. 

Phytoflagellata,  colony  formation,  40, 

42. 
,,  evolution  of  sex,  89 

—91. 

Pierinae,  345. 
pig,  feet,  241,  Fig.  98. 
pineal  eyes,  258—260,  Figs.  114—116. 

„  gland,  11,  260. 
pin-eyed  flowers,  356. 
pistil,  106.  t 

Pisum  sativum,  Tarilties,  196. 
pitcher  plants,  262. 
Pithecanthropus  er^ctus,  423. 
pituitary  body,  408. 
placenta,  270. 
placental  mammafe,    appearance  of, 

302. 

Plagianthus  bctuliims,  417. 
Plagiaulacidse,  302g 
Plagiaulax,  302,  1%  148. 
Planema  poggei,  346. 
plankton,  322. 
plants,  compared  with  animals,  34,  35. 

„      dispersal  of,  321. 
plasma,  51. 
plasmodia,  67. 
plasmogamy,  128. 
plasticity  of  organisms,  335. 
plastids,"  32.  > 

plastogamy,  128. 
Platypus  (see  Ornithorhynchus). 
Pleistocene  Epoch,  284. 
plerome,  50. 

Plesiosauria,  236,  244,  297. 
Plesiosaurn*  macrocephaltts,  Fig.  HI. 
Pliny,  262. 
Pliocene  Epoch,  284. 
Pliohippus,  310,  311,  Figs.  153,  158. 


ploughshare  bone,  305. 

plumage  of  birds,  349. 

plumcots,  204. 

pluteus  larvae,  275,  276,  Fig.  127. 

polar  bodies,  133,  135, 139,  145,  Figs. 

35,  65,  68. 

pole-cell,  in  insect  egg,  130. 
pollen,  collection  of,  355. 
„       grains,  106,  107. 
,,       sacs,  107. 
tube,  108. 
pollination,  110,  111. 

,,          adaptations  for,  351 — 363. 
pollinia,  361. 

polyanthus,  pollination,  356. 
polydactylism,  149. 
polymorphic  colony,  119. 
polymorphism  in  Papilio,  346,   Fig; 

177. 

polype,  fresh  water  (see  Hydra). 
Polypodium,  archegonium,  Fig.  52. 
Polyzoa,  dispersal,  327. 
population,  Buffon  on,  368. 

„          Charles  Darwin  on,  386. 
„          Lamarck  on,  381. 
„          Wallace  on,  389. 
porpoises,  convergence  in,  248. 
Portunus,  247,  Fi<r.  102. 

,,         larva,  Fig.  128. 
postaxial,  238. 
potential  energy,  7. 
Poulton,  E.  B.,  336,  337,  346. 
preaxial,  238. 

Pre-Cainbrian  Epoch,  284,  285. 
pre-formation  in  development,  163. 
prehension,  organs  of,  256. 
prepotency,  353,  358. 
presence  or  absence  hypothesis,  207. 
Primary  Era,  284,  285. 
Primates,  301,  303. 
primordia,  in  germ  plasm,  206. 
primordial  germ  cells,  113  (see  alsc 

germ  cells,  origin). 
„  utricle,  62,  95. 

Primula,  fertilization,  356 — 358. 
priority  in  nomenclature,  227. 
Proboscidea,    evolution,    312  —  314, 

Fig.  159. 
proboscis,  of  bee,  354,  Fig.  179. 

„  butterflies  and    moths, 

354. 

,,  elephants,  256. 

,,  insects, Erasmus  Darwin 

on,  373. 
„  mutual  adaptation, 

414,  415. 
procryptic  colouration,  337—342. 


448 


INDEX 


progress,  human,  426,  427. 
progressive  development,  E.  Cham- 
bers on,  384. 
,,  evolution,  334. 

,,  cause  of,  192. 

Wallace  on,  390. 
series,  232,  234. 

pronuclei,  131,  145,  146,  Pig.  64. 
protandrous,  353,  363. 
protective  resemblance,  337 — 342. 
,,  and  fluctuating  varia- 

tion, 415. 
,,  ,,    natural  selection, 

396,  405. 
proteids,  7,  22. 

Proterotherium,  253,  Fig.  109. 
Proteus,  183. 
prothallus,  of  fern,  103. 

,,          ,,  flowering  plant,  108. 
Protista,  38. 

,,       dispersal  of,  327. 
,,       immortality  of,  161. 
protogynous,  353. 
Protohippus,  310. 
Protophyta,  38,  95. 

, ,          sexual  differentiation  in, 

91. 
protoplasm,  2,  38. 

„  chemical     composition, 

22,  23. 

controlling  power,  23. 
energy  in,  7. 
physical  properties,  21. 
selective  action,  26. 
sensitive  to  stimuli,  188. 
streaming,  62. 
Protopterus,  255,  291. 
Protorohippus,  310,  Fig.  153. 
Prototheria,  301,  302. 
Protozoa,  38,  &c.,  Figs.  3,  4,  9,  &c. 
,,         non-adaptive     characters, 

419. 

,,        nuclear  reduction,  139. 
,,        powers  of  multiplication, 

81. 

,,        sexual  differentiation,  89. 
Prozeuglodon,  318,  Fig.  163. 
prunes,  stoneless,  204. 
Pseudopanax  cliathamicum,  417. 
„  crassifolium,  417. 

„  ferox,  417. 

pseudopodia,  15. 
Pteranodon,  299,  Fig.  147. 
Pteraspis,  290. 
Pterichthys,  290,  Fig.  136. 
Pterosauria  (pterodactyls),  236,  242, 
299,  Figs.  99,  147. 


Pterostylis    trullifolia,    fertilization, 

362. 

Pterygotus  osiliensis,  Fig.  137. 
PulchripJiyllium     crurifolium,     338, 

Fig.  170. 

Punnett,  E,  C.,  208. 
purity  of  gametes,  200,  206. 
putrefaction,  218. 

Pyrenees,  arctic  vegetation  ou,  333. 
pyrenoids,  32,  Figs.  5,  42. 

a 

QUADRTTMANA,  424. 
quartz,  24. 

E. 

EABBIT,  embryo,  265,  Fig.  117. 

„        shoulder  girdle,  234,  Fig.  90 , 
racial  characters,  transmission,  175. 
radial  canals,  120. 
Eadiolaria,  25,  Fig.  3. 
radius,  237. 

rafts,  dispersal  by,  324. 
range  in  time  of  animal  groups,  284. 
Ranunculus  aquatilis  and  Ranunculus 

hederaceus,  Lamarck  on,  379. 
rats,  179,  182,  325. 
recapitulation  hypothesis,  265 — 281. 
„  ,,        anticipated 

by  Erasmus  Darwin,  372. 
receptor,  18. 

recessive  characters,  197,  207. 
recognition  marks,  337. 
red  blood  corpuscles,  52,  53,  Fig.  15. 
„   clover,  robbed  by  humble  bee, 

355,  356. 

Eed  Sea,  fauna,  323. 
red  snow,  27. 

reduction  of  chromosomes  (reducing 
division),  132—139,  206,  207,  Figs. 
65—68. 

regeneration,  167. 
regression,  filial,  210. 
rejuvenescence,  92. 
reproduction,  10. 
Eeptilia,  compared  with  Amphibia, 

294. 
„         compared  with  Mammalia, 

232,  235. 

„         fossil,  295—299. 

,,         geological  range,  284. 

giant,  406. 

limbs,  235. 

„        origin,  294,  295. 


INDEX 


449 


respiration,  8. 

,,          in  Amoeba,  17. 

„  „  embryos,  270. 

„  ,,  plants,  32,  33. 

response  to  stimuli,  18. 
Reunion,  peach  trees  in,  182. 
reversion,  261,  262. 

„          Mendelian      explanation, 

208. 

rhinoceros,  feet,  241. 
Rhynchocephalia,  259,  295  (see  also 

Sphenodon). 
ribbon-wood,  417. 
ribs,  meristic  variation,  149. 
right  whale,  245,  Fig.  101. 
Eitzema  Bos,  179. 
rock-formation,  282. 
rodents,  301. 
"Rodney, "brig,  399. 
Rodriguez,  258. 
Romanes,  G.  J.,  333,  417,  418. 
Rontgen  rays,  187. 
root-cap,  50. 

root,  growing  point,  76,  Figs.  33,  34. 
Rosenthal,  179. 
rostellum,  360. 

rotifers,  parthenogenesis,  144. 
rudimentary    organs     (see    vestigial 
organs). 

,,  ,,       Charles  Darwin 

on,  392. 

Rumia  cratcegata,  larva,  Fig.  169. 
rust,  in  wheat,  205. 
rye,  fertilization,  412. 


S. 


SACCULINA,  degeneration,  401,  Fig. 

183. 

sage  (see  Salvia). 
Sa<ritta,  origin   of  germ  cells,   130, 

Fig.  63. 

salt,  in  sea,  287. 
Salvia,     structure     of     flower     and 

fertilization,  358,  359,  Fig.  181. 
Salvinia,  105. 

Sandwich  Islands,  oceanic,  332. 
sap,  65. 

,,    nuclear,  70. 
saprophytic  nutrition,  34. 
sarcode,  38. 
Sarracenia,  262. 
Sauropsida,  300. 
Sauropterygia,  297,  Fig.  141. 
scapula,  234,  Figs.  89,  90,  93. 
scents,  of  flowers,  355. 

B. 


Schleicher,  69. 
Schleiden,  38. 
Schoeteusack,  423. 
Schwann,  38. 
Scrophularia  nodosa,  353. 
scurf,  55. 

sea  anemones,  114. 
,,  coast,  selection  in,  395. 
„  firs,  114. 

„      urchins,     artificial     partheno- 
genesis, 145. 

„  „  blastomeres,  45. 

„  „  hybridization,      174, 

175. 

„  „  larvae,  276. 

seal,  paddles,  244,  Fig.  100. 
Secondary  Era,  284,  285. 

,,          sexual      characters       in 

Coelomata,  125. 
secretions,  controlling  growth,  408. 

„          internal,  126. 
sedimentary  rocks,  282,  286. 
sedimentation,  rate  of,  285,  286. 
seed,  109,  Fig.  56. 
„     coat,  109. 
,,     leaves  (see  cotyledons), 
seedling  of  Acacia,  280,  Fig.  132. 

„  pea,  Fig.  56. 
seeds,  dispersal,  321. 
Seeley,  H.  G.,  297. 
segmentation,  metameric,  149,  266. 
segmentation  of  ovum,  45,  263. 
in  Amphioxus,  45,  Fig.  13. 
,,  birds  and  reptiles,  270. 
„  frog,  268,  Fig.  119. 
„  Hydra,  118. 
,,   mammals,  271. 
interpretation  of,  265. 
segregation  of  germ  cells  (see  germ 

cells,  origin], 
selection,  artificial,  395,  396. 

,,  ,,       Charles    Darwin 

on,  387. 

„         by  insects,  396. 
,,         continuous,  411. 
„         germinal,  173. 
„        natural  (see  natural  selec- 

•  tion). 
„         not     confined     to     living 

things,  395. 

„        sexual    (see   sexual  selec- 
tion). 

„         single,  412. 
self-fertilization,  351,  352,  353. 
sematic  colours,  336,  342. 
Semon,  Richard,  186. 
sensation,  16. 

G  a 


45.0 


INDEX 


sense  cells,  stimulation  of,  188. 

„     organs,  18,  121. 
sepals,  106. 

series,  progressive,  232,  234. 
Sesia  crabroniformis,  Fig.  175. 
sex  determination  in   insects,    135, 

Fig.  67. 
sexes,       fundamental       distinction 

between,  127. 

sexual  characters  transferred  to 
asexual  generation,  110, 
111. 

,,      differentiation,  origin  of,  128. 
sexual  phenomena,  84,  85. 
evolution  of,  126,  127. 
in  Bodo,  87,  Fig.  38. 
Coccidium,  88,  Fig.  39. 
Coelomata,  125. 
Copromonas,  83,  Fig.  37. 
Eudorina,  91,  Fig.  40. 
ferns,  104,  105,  Figs.  51,  52 
flowering     plants,     108,    109, 

Figs.  54,  55. 
Fucus,  100,  101,  Fig.  47. 
Haematococcus,  33,  Fig.  5. 
Hydra,  116. 
Obelia,  121. 

Pandorina,  89,  Fig.  10. 
Paramoecium,  93,  Fig.  41. 
Spirogyra,  96,  97,  Figs.  43, 44. 
Volvox,  91,  Fig.  11. 
Zygogonium,  97. 
sexual  reproduction,  33. 

„      selection,  Charles  Darwin  on, 

388. 
„  „        Erasmus      Darwin 

on,  373. 

Shand,  Alexander,  399. 
sharks,  fossil,  291. 
sheep,  Ancon  or  otter,  154. 

„      feet,  241. 

shepherd's  purse,  48,  Fig.  14. 
Shirreff,  Patrick,  412. 
shoulder  girdle,  of  mammals,       234, 

Fig.  90. 
,,  ,,       ,,  Ornithorhynchus, 

234,  Fig.  93. 
„  ,,       ,,   reptiles,  233,  234, 

Fig.  89. 

Sicilian  pea,  208. 
silica,  23,  24. 
siliceous  skeleton,  25. 
Silurian  Epoch,  284. 
single  selection,  412. 
Sirenia,  301. 

paddles,  236,  244. 
size,  increase  of,  309,  405—409. 


skeleton    (see    feet,    limbs,    paddles 


of 


Figs. 


extinct    animals, 

139—152. 

„    giraffe,  Fig.  104. 
,,    horse  and  man,  Fig.  95. 
„   kiwi,  Fig.  112. 
„   Obelia,  119. 
„   Protista,    23—26,   Figs 

3,4. 

„         „   whale,  Fig.  101. 
skulls    of    dog  and  thylacine,   251^ 

Figs.  107,  108. 
slime  fungi  (see  Mycetozoa). 
sloths,  333. 

slow  worm  (see  Anguis  fragilis}. 
snails,  dispersal,  325. 
snakes,  mimicry  in,  343. 
social  problems,  de  Vries  on,  414. 
sodium,  in  sea  water,  287. 
sole  of  foot,  Buff  on  on,  368. 
solitaire,  258,  397. 
Sollas,  W.J.,  286,287,311. 
soma  and  germ  cells  contrasted  (see 

germ  cells  and  somatic  cells), 
somatic  and  germ  nuclei  in  Para- 

mcecium,  98. 

somatogenic     characters,     supposed 
non-inheritance,  169,  176  (see  also 
acquired  characters), 
somatogenic    variations,    149,    156, 

157. 

somatopleure,  124. 
somites,  266. 
soul,  11,  19. 
South  America,  fauna  and  flora,  328, 

329,  333. 

Spalacotherium,  302. 
special  creation,  212—214,  222,  Fig. 

82. 

„  Buff  on  on,  369. 

„  Lamarck     on,    375, 

376. 


speces, 


ggregate,  224. 
harles  Darwin  on, 


223. 


on, 


definition  of, 
elemeutary,  23 
Lamarck  on,  375. 
modification  of,   Buffon 

366,  367. 

nature  of,  222—224. 
number  of  liviug,  211. 
origin  from  mutations,  225. 
supposed  immutability  of  (see 

immutability  of  species). 
transformation  of,  Lamarck 
on,  376,  377. 


INDEX 


451 


specific      characters,     non  -adaptive, 

419—422. 
speech,  an  acquired  character,    156, 

184. 

„       evolution  of,  425. 
Spencer,  Herbert,  2,  185,  391. 
spermary,  113. 
spermatids,  133,  Figs.  65,  66. 
spermatist  (Erasmus  Darwin),  371. 
spermatocytes,  133,  Figs.  65,  66. 
spermatogenesis,  132,  133,  134,  135, 

Figs.  65,  66. 

spermatogonia,  133,  Figs.  65,  66. 
spermatozoa,  85,  139,  Fig.  69. 

,,  chemotaxis  in,  141. 

,,  development   (see  sper- 

matogenesis). 
of  Chara,  141. 
„  „  Coccidium,  88,    141, 

Fig.  39. 

„  Eudorina,91,Fig.40. 
„  „  fern,  104,  141,  Fig. 

51.  : 

„  „  Fucus,  100,  Fig.  47. 

„  Hydra,  116. 
„  ,,  medusae,  121. 

,,  ,,  mosses,  141. 

„  sponges,  113. 
„  Yolvox,  91,  Fig.  11. 
spermatozoids,  91. 

sphserechinus,  experiments  in  hybri- 
dization, 174. 

Sphaerella  (see  Hsematococcus). 
Sphserozoum,  43,  265,  Fig.  12. 
Sphenodon  punctatus,  259,  260,  295, 

320,  331,  Figs.  113,  114. 
spicules  (see  sponges,  spicules). 
spider  crab,  339,  340,  Fig.  172. 
spindle,  nuclear,  71. 
spines,  excessive  development,  406. 
spiny  anteater  (see  Echidna), 
spireme,  71. 

Spirogyra,  95—97,  142,  Figs.  42—44. 
splanchnopleure,  124. 
splint  bones,  241,  257. 
sponges,  deep  sea,  335. 

„        dispersal   of    fresh    water, 

327. 
„        gemmules  of  fresh  water, 

327,  Fig.  166. 
„  germ  cells,  113. 
„  non  -  adaptive  characters, 

419—421. 
„        spicules,  26,  232,  419,  420, 

Figs.  88,  187. 

Sponf/iUa  fluviatilis,  gemmules,  Fig. 
16(3. 


spontaneous  generation,    214 — 216, 

219. 

Lamarck  on,  374. 
sporangia  of  fern,  101,  Fig.  48. 

„        „  flowering    plant,     107, 

Fig.  53. 
spores,  101. 

,,      dispersal,  321. 

„       formation,  reduction  of  chro 

mosomes,  138. 

„       germination,  in  fern,  103. 
„       of  Bodo,  87. 
„       ,,  Coccidium,  89. 
sporophylls,  107. 
sporophyte,  101. 

,,    '      of  fern,  105,  Fig.  48. 
„  ,,  flowering  plants,  106. 

„          number  of  chromosomes, 

139. 

sports,  154,  224  (see  also  mutations). 
Squalodontidse,  318. 
Squamata,  295. 

stability  of  organisms,  155,  Fig.  74. 
stag,  antlers,  125. 
staining  reactions,  62,  70. 
stamens,  106. 
staminodes,  262. 
starch,  31. 

star-fish,  meristic  variation,  149. 
statoblasts,  327,  Fig.  165. 
stature,  variation  in  human,  152. 
Staurocephalus  murchisoni,  Fig.  134. 
Stebbing,  T.  E.  E.,  339. 
Stegocephalia,  293. 
Stegosaurus,  299,  Fig.  145. 
Stendbothrus      viridultis,     spermato- 
genesis, Fig.  66. 
Stentor,  Fig.  9. 
Stereognathus,  302. 
sterility    of    species    when,    crossed, 

418. 

St.  Helena,  an  oceanic  island,  332. 
stick     caterpillars,    337,    338,    Fig. 

169. 

Stieda's  organ,  260,  Figs.  115,  116. 
stigma,  of  flower,  107. 
stimulation  of  protoplasm  and  cells, 

188. 
stimuli,  18. 

„      to  development,  145,  146. 
stimulus,  transmission  between  cells, 

143. 
„  „  to  germ  cells, 

186—190. 
Stockard,  157. 
stomata,  64,  Fig.  26. 
storms,  dispersal  by,  324. 


452 


INDEX 


stratified  epithelium,  56,  Fig.  18. 

rocks,  282—286. 
striped  muscle,  57,  Fig.  22. 
struggle  for  existence, 

A.  E.  Wallace  on,  389. 
Buffon  on,  368. 
Charles  Darwin  on,  386. 
importance  of,  397,  398,  400. 
Lamarck  on,  381. 
stump  tails,  179. 
style,  of  flower,  107. 
StyUdium  graminifolia,  fertilization, 

362. 

subclasses,  226. 
subfamilies,  226. 
subgenera,  225. 
subkingdoms,  226. 
suborders,  226. 
substantive  mutations,  154. 

,,  variations,  148,  150. 

succulent  plants,  350. 
Summer,  F.  B.,  182. 
sun,  dependence  of  life  upon,  o. 
suprascapula,  234. 
surface,  relation  to  volume,  20. 

,,        tension,  19. 
survival  of  fittest,  391. 

„  ,,      Buffon  on,  368. 

,,  ,,      Charles  Darwin  on, 

387. 
„  „      in  inorganic  world, 

395. 

suspensor,  49. 
Svalof,  412. 

sweet  peas,  reversion  in,  208. 
swim  bladder  and  lungs,  255,  256. 
synaposematic     groups,      343 — 346, 

Fig.  176. 
synapsis,  133,  137,  206,  Figs.  65,  66, 

68. 

synaptic  mates,  135. 
syncytia,  66. 

syncytial  epithelium,  67,  Fig.  28. 
syndesis,  137  (see  also  synapsis). 
syngamy,  83  (see  also  conjugation), 
systematist,  work  of,  226. 

T. 

TADPOLE,  ascidian,  277,  Fig.  130. 

frog,  272,  Fig.  121. 

stage,  278,  279. 
tail,  in  man,  261,  262,  424. 
tails,  mutilation  of,  178,  179. 
tapir,  feet,  241,  Fig.  96. 
tarsals,  237. 
taxonomic  tree,  228,  229,  Fig.  86. 


taxonomy,  226. 

„          aims  of,  229. 
teeth,  of  Cetacea,  317,  318,  Figs.  162, 

163. 
„       „  dog   and    thylacine,    251 — 

253,  Figs.  107,  108. 
„       ,,  elephants,  314. 
„       ,,  horses,  309. 
,,     vestigial,  260. 
Teleostei,  292. 
telescoping  of  generations,  111,  112, 

122. 

temperature  of  body,  6. 
tentacles,  115,  118. 
Tertiary  Era,  284,  285. 
testis,  113,  116. 
Tetrabelodon  angustidens,    314,   Fig. 

159. 
„  longirostris,    314,    Fig. 

159. 
Tetraxonida,  spicules,  232,  Figs.  88, 

187. 

Theosodon,  253,  Fig.  109. 
Theromorpha,  296. 
Thoatheriurn,  253,  Fig.  109. 
thrum-eyed  flowers,  356. 
thylacines,  301. 

Thylacinus,  251,  Figs.  107,  108. 
tibia,  237. 
time,  geological,  285—287. 

„     importance  in  evolution,  Buffon 

on,  366. 

,,     in  evolution  of  horse,  311. 
,,     scale,  geological,  284.    * 
Tinoceras,  303,  406,  Fig.  150. 
Tintinnopsis,  Fig.  9. 
tissue -formation,  45,  50,  51,  75 — 79. 
tissues,  37,  51—65. 
Tithorea  harmonia,  345,  Fig.  176. 
tools,  use  of,  424,  425. 
torpedo,  256. 
tortoise,  334. 

Tower,  W.  L.,  156,  159,  184. 
Tradescantia,    histology   of,    61 — 65, 

Figs.  25,  26. 
transformation      of      species      (see 

species), 
transparency,    of    pelagic    animals, 

322,  336. 
tree-like  classification,  228,  229,  Fig. 

86. 

„         evolution,  221. 
Trematoda,  parthenogenesis,  144. 
Triassic  Epoch,  284. 
Triceratops,  299,  Fig.  146. 
trichocysts,  39. 
Triconodon,  302. 


INDEX 


453 


trilobites,  289,  Fig.  134. 

Trimen,  E.,  346. 

Triticum  sativum,  Lamarck  on,  378. 

Tritylodon,  301,  302. 

tuatara  (see  Splienodon  pundatus). 

Tubularia,  121,  Fig.  61. 

turbellarian,    fertilization    of    polar 

body,  135. 

turbot,  colour  changes,  342. 
turtles,  dispersal,  323 

„       limbs,  236,244. 
twins,  identical,  173. 
types,  in  wheat,  412. 
Tyndall,  215. 
Typhlopidee,  250. 


ULNA,  237. 

unconscious  memory,  191,  192. 

ungulates,  301. 

limbs  of,  239—241. 
unguligrade,  240. 
unicellular,  38. 
unisexual,  99,  105. 
unit  characters,  200,  206. 

„  origin  of,  415,  416. 

unstriped  muscle,  57,  Fig.  21. 
Uroeotyplilus  africanus,  Fig.  106. 
urea,  9. 
Ursus  arctos,  systematic  position,  229, 

Fig.  86. 
use  and  disuse,  effects  of,  156,  166. 

Buffon  on,  367—368. 

Charles  Darwin  on,  391,  392. 

Lamarck  on,  377,  378,  379,  380, 
382. 

E.  Chambers  on,  385. 
uterus,  125. 


VACUOLES,  contractile,  39,  83. 
Varanus,  shoulder  girdle,  234,  Fig. 

89. 

variation,  148—160. 
„        cause  of,  166. 

curve  of,  150, 153,  Figs.  72, 

73. 
„         in   grape    hyacinth,     150, 

Fig.  72. 
,,          ,,    human    stature,     152, 

Fig.  73. 
variations,   blastogenic,    149,    157 — 

160. 

„          congenital,  157. 
,,          continuous,  148,  150. 


variations,  ""discontinuous,   149,    153 

—156. 
,,          fluctuating,      148,      150, 

155,  Figs.  72,  73. 
germinal,  149,  157—160. 
,,  individual,  150. 

inheritance,  403,  404. 
meristic,  148,  149. 
,,  normal,  150. 

origin,     153,    173,    403, 

404. 

selective  value,  403,  404. 
,,  somatogenic,    149,    156, 

157. 

„  substantive,  148,  150. 

vasa  deferentia,  125. 
vascular  bundles,  65. 

„         system  in  plants,  50. 
vegetable  marrow,  flowers,  111. 
vegetative  cell  in  pollen  grain,  108. 
Vergil,  214. 

vertebrae,  meristic  variation,  149. 
vertebral  column,  development,  266. 
VertebfataT-geol  ogieaL Jnstory,   289 

—304. 

„    "       origin,  291. 
Vespa  crabro,  Fig.  175. 
vessels,  of  plants,  65. 
vestigial  organs,  257—262,  271,  345, 

397. 
,,  ,,       Charles  Darwin  on, 

392. 

vibrations,  in  determinants,  189. 
Yirchow,  38. 
vital  force,  11,  143,  216. 
vitalism,  19. 

yitelline  membrane,  140. 
volume,  increase  of,    Lamarck  on, 

382. 

„         relation  to  surface,  20. 
Yolvox,  42,  91,  97,  98,  265,  Fig.  11. 
Von  Baer,  265. 
Von  Mohl,  38. 
Vvrticella,  40,  Fig.  9. 

W. 

WAISTS,  slender,  156. 

Walker,  C.  E.,  175. 

Wallace,  A.  E.,  views  of,  319,  329, 

389—391,  393,  394. 
warm-blooded  animals,  6. 
warning  colours,  342 — 348. 

,,  ,,         and  natural  select 

tion,  396. 

wasps,  colours,  342. 
waste  products,  8.  9. 


454 


INDEX 


wattles,  life-history,  280. 

weapons,  use  of,  424,  425. 

weeds,  dispersal,  325. 

Weismann,  views  of,  146,  166—174, 

176,  177,  178,  179,  180,  1537189, 

206,  219. 
weka,  397,  398. 
Wells,  W.  0.,  385. 
Western,  387. 
w hales,  ancestry,  314 — 318. 

,,       cervical  vertebrae,  248,  Fig. 
103. 

„       convergence  in,  248. 
paddles,  244,  Fig.  101. 

„       size,  303. 

„       systematic  position,  301. 

„       vestigial  organs  in,  257,  260, 

Fig.  101. 
wheat,  improvement  of,  411 — 413. 

,,      Lamarck  on,  378. 
wind,  dispersal  by,  323. 
winged  animals,  dispersal  of,  323. 
wings,  236. 

„     convergence  in,  248 — 250. 

„     of  bat,  243,  Fig.  99. 

„      „  bird,  243,  244,  Fig.  99. 

,,      ,,  insects,  247. 

,,      „  pterodactyls,  242,  Fig.  99. 
wireless  telegraphy,  analogy  of,  187. 
Wolfe,  C.F.,  163. 
wombats,  301. 
wood,  65. 


Woodward,  A.  S.,  253, 

301,  302. 
worms,  dispersal,  325. 


296,  300, 


X. 


XYLEM,  65. 


Y. 

YOLK,  140,  267. 

„     corpuscles,  117. 

„    influence  on  development,  268 

—270. 
„    sac,  270,  271,  Fig.  120. 

Z. 

ZEUGLODONTID^E,  318. 

zoaea  larva,  276,  Fig.  128. 

zona  radiata,  140. 

zooids,  43,  119. 

zoophytes,  34,  119. 

zoospores,  29. 

Zoothamnium,  40. 

Zoster  ops  later  alis,  dispersal,  324. 

Zygogonium,  conjugation,  97,  143. 

zygosis,  83,  131. 

zy  go  spore,  97. 

zygote,  33.   83,  87,  88,  91,  97,  101, 

105,  109,  118. 

„        conjugation    of    nuclei   in, 
131,  Fk'.  64. 


BBADBUBY,   AGNEW,    &    CO.   LD.,   PBINTEBS,   LONDON   AND    TONBBIDGE. 


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